Monster refers to the more animal-like characters you'll find, and include characters like Prinnies or Wood Golems. A weakness unique to all monsters is the lack of an ability to lift or throw other characters. Monsters can only equip monster-exclusive weapons, and thus do not have the Weapon Mastery statistics of Humans. Story characters such as Tink or Hanako are not included, as they are thought to be separate, unique classes.
To unlock the first tier of monster characters, defeat a monster of that type in any stage (the " Unlock" category of first tier monsters will list the story stage they can first be found in). Monsters defeated in the Item World do not count. Advanced tiers cannot be unlocked this way. Characters must be advanced to an appropriate level to unlock the next tier, or be captured at a high enough level to unlock the next tier. You cannot capture a monster you cannot already create.
In the first cycle, Ghost SP-recovery can come in handy. In the post-game, they have the highest INT of monsters. They're sort of a gimmick at either point, since human magic-users are generally better because of staff mastery, and having SP doesn't matter that much until the post game.
If you have an all-monster party, you're going to need one of these as a spellcaster, and you'll want to get it some of the cross-class spells. You shouldn't need a Ghost otherwise.
Undead
Ability: Completely recovers HP and SP after killing an enemy with a normal attack.
Lover: Instead of the typical 1% increase, Lovers give a 1.25% increase to all stats.
Stats are poor, aptitude is poor, HP is good, but Zombies are mostly poor. Even so, that innate ability has amazing potential if you can find ways to use it regularly.
Succubus
Ability: Inflicts 25% more damage to male characters.
Lover: Inflicts an additional 1% damage to male characters.
Succubus are tricky little devils, and the only ones that deal more damage to humans, albeit only males. Unfortunately half of the generic humanoid classes are female: Girl Mages, Thieves, Lady Fighters, Kunoichi, Rune Knights, Beastmasters, Clerics, and Archers. But on the bright side only two of those classes usually carry much in the way of DEF or HP, so the Succubus isn't missing much by catering mainly to men.
Their specials are easily the most varied from monster selections, and not only are they the only class capable of sapping HP, they are one of only three monster that can naturally heal allies. The drawback: their specials focus on three different stats, which makes it less effective to focus on a single strength.
Useful as they may be, Succubus lack much strength in any single area. If you're a strategist, give 'em a try. If you just like power-leveling and beating everything into the ground with pure stats, don't bother.
It's too bad monsters can't equip fists, because Felynns would be perfect for them. But even without fists, they can handle themselves pretty well. Where they truly shine is attacking and counterattacking; the ability should make it obvious, but COUNTER, ATK, and SPD are all high.
Before you get Wood Golems and Rifle Demons in Episode 13, these are probably your best warrior-monsters.
Dragon
Ability: Immune to fire attacks.
Lover: When attacked with fire damage, heals 1% of the damage per lover (similar to the Reverse Damage Geo Effect).
Dragons make for decent warrior-type monsters until you get Wood Golems and Rifle Demons in Episode 13. Their Fire area-of-effect physical specials can be useful, but you'll probably have that base pretty well covered with Adell.
When creating one of these guys, make sure you aim for -50% fire. Not only will the Dragon lack weaknesses, but if you get lovers for him, he'll absorb more than if he had 0% or 50%.
Prinny
Ability: Explodes when thrown, dealing area-of-effect damage equal to half its current HP. An exploding Prinny will detonate other nearby Prinnies. (Prinny-throwing does not count as an ally kill). Fully healing Prinnies at the Hospital only costs 1 HL.
Prinnies... are weak, and they were meant to be weak. They might have the worst stats and aptitude of any playable character in the entire game. However, their real strength lies in the Dark Assembly bill "Prinny Day", which vastly increases the chance of Rare and Legendary items appearing in the bonus list, and Etna's Prinny-boosting ability, and of course their innate abilities.
When, or if, you're not using them to attack directly, they work well as grenades. Because of chain-exploding, with nine Prinnies you can easily clear the battlefield for a Majin's ability.
If you're using them to attack, put everything into ATK. If you're using them as explosives, dump everything into HP.
Don't underestimate these doods. Yeah, because when they're many, their team attacks could kill you immediately. But that's not enough, right? There's a secret in a prinny (well not much of a secret) when Etna's around, they're very strong. So if you're going to use a prinny party, have Etna.
Orc
Ability: When critical, increases ATK by 25%.
Lover: When critical, increases ATK by an additional 1% per lover.
Orcs are good attackers when you get them, and can be among your best throughout the game if you used properly. Their innate can be unleashed with a weight, and that 25-50% boost may or may not be worth the risk, depending upon your situation. But it's really all about about Orc God after Lv65: it covers a huge 3x3 area, has amazing range considering, and boasts very impressive strength.
Winged
Ability: While in danger, move range increases by one. Wingeds can move through enemy characters (like Tink and White Dragons).
Lover: While in danger, move range increases by one for every ten lovers.
Stats? Aptitude? Horrendous. These might as well only exist for movement and jump. In the Item World, sometimes that's all you'll need, and that's when you'll bring one of these out with a weight and two shoe-type items. Their ability to move through enemies only helps that attribute of them more.
If you're actually daring enough to want to use these in battle, their specials are their best aspect. One has a good chance of sleep on up to three enemies, while another does the same with poison. Tornado Drive is strong, but with a Winged's pathetic ATK, don't expect much damage.
Aqua Demon
Ability: Immune to Ice attacks.
Lover: When attacked with ice damage, heals 1% of the damage per lover (similar to the Reverse Damage Geo Effect).
They're... okay, not really worth having over most magic-users, but they can defend against physical attacks better than other wizard-types. Plasma Charge can help out, but given they have to waste a turn using it on themselves, that massive 50% boost suddenly seems less impressive. If you just want a magic-using monsters, just hold out a couple episodes for a Succubus.
When creating one of these, aim to get Ice at -50%. Not only will this take away any elemental weaknesses, but you'll heal more from Ice attacks if you ever get an Aqua Demon lover.
Flora Beast
Ability: Takes 50% less damage from magic attacks.
Lover: Takes 1% less damage from magic attacks per lover.
They'll survive virtually any magic attack, but the range on most magical attacks is so good it's hard to force enemies to target Flora Beasts when you need them to. The only other thing they can do is heal... but Clerics, White Dragons, and Flonne are better at it. So... you probably won't want these.
Mystic Beast
Ability: Immune to wind attacks.
Lover: When attacked with wind, heals 1% damage per lover (similar to the Reverse Damage Geo Effect).
They're decent in most areas, but lack any stand-out reason to have them in your party. If you're going to keep one, it will probably be due to their high mobility and wind immunity. But there are better monsters, some of which even level up faster... Avoid making these unless you need it.
When creating one of these, aim to get Wind at -50%. Not only will this take away any elemental weaknesses, but you'll heal more from Wind attacks if you ever get a Mystic Beast lover.
Marionette
Ability: Immune to status ailments (paralyze, poison, etc).
Lover: Instead of the typical 1% increase, Lovers give a 1.25% increase to all stats.
Of the 3-EXP-Rate characters, these are easily have the best stats overall, but are inferior to most slower-leveling characters. Their ability will be useful sooner or later, especially when facing Wingeds. You'll be better off with a Marionette in the first cycle, but their stats and ability will seem less vital in the post-game.
Holy Dragon
Ability: Immune to non-elemental special moves. Holy Dragons can move through enemies, Geo Symbols, and chests.
Lover: When attacked with non-elemental special moves, heals 1% of the damage per lover (similar to the Reverse Damage Geo Effect).
Stats, Aptitude? Horrible, especially considering their EXP Rate. The only reason you'll want these is their ability, and the special which cures and heals simultaneously. The only other special which can do the same belongs to Flonne. If neither of those attributes sound amazing to you, don't bother with these.
Rifle Demon
Ability: Attacks never miss.
Lover: Instead of the typical 1% increase, Lovers give a 1.25% increase to all stats.
Attacks never miss. That would be enough reason to make these legendary, but their ATK and HP are godly too. The only possible "drawbacks" are you can't get them until episode 13, they aren't the most agile bunch, and their specials are on the costly side. Even if you never use monsters, you'll need one of these.
Wood Golem
Ability: Heal 20% of HP at the beginning of each round.
ATK and HP are off the charts, and that ability is godly. These are indisputably the best monsters for attacking, but their low SP and HIT are a small flaw in an otherwise perfect sculpture. Unfortunately you can't get these until the last episode. But immediately after you kill one, create one (or two).
Dragon Zombie
Ability: Takes 50% less damage from physical attacks.
Lover: Takes 1% less damage from physical attacks per lover.
Dragon Zombies are all about DEF, but their ATK isn't too bad either. However, their RES is abysmal for a defensive unit, to the point where only a handful of magical attacks could take these guys out. They can be useful, but it's easy to argue defensive units aren't always worth it--especially considering Dragon Zombies are monsters, so they can't lift and protect weaker allies like Heavy Knights can.
Roc
Ability: Reduces the stats of all adjacent enemies by 20%.
Lover: Enemy stats decreased by an extra 1% per lover.
Stats are half-decent, but Rocs are all about their innate ability. Surround a foe with one or two, attack, move cancel the Rocs back into place, then repeat on another enemy. It's also a huge boost when you're just attacking the foe with the Roc itself (especially with enough Lovers). Note that however many Rocs are surrounding a foe, you can't reduce their stats more than 50%.