Mount&Blade/Neutral

In addition to the various factions there are also some party types that aren't affected by faction relations.

Mercenaries
Troops from the mercenary line are most easily found in taverns, but they can also be acquired by attacking caravans and then convincing the surviving Caravan Guards to join you. You can also rescue them from parties that have destroyed caravans. Faction parties will regularly attack any caravans that belong to their enemies, so it is quite common to find a faction party carrying a few Caravan Guards. Mercenaries should be used sparingly to supplement an army intent on besieging a town or castle or to fill in the roles of a particular unit a faction is lacking in. Their higher upkeep costs make them unattractive to most adventurers or captains but as expendable soldiers, they are abundant and put up a reasonable fight. After all, you don't pay upkeep for dead mercenaries.

Bandits
Bandit parties are found roaming various parts of the map. They will attack any party smaller than their own, except for other bandits. Some bandits do not upgrade at all, while others can eventually upgrade into Mercenaries if you manage to keep them alive.

Often, bandits freed / captured and recruited, upgrades from something such as "steppe bandit" will be to Kerghit Tribesman, whereas Sea Raiders can become Nord Recruits - if you'd swop a bunch of armoured, axe throwing Vikings for some Nordic farmer who has the potential (after a lot of time, money and effort) to become a better armoured, axe-throwing Viking or die trying...

Sometimes, it is best to just use Forest Bandits as extra, cheaper arrows in a garrison (as opposed to, say, skirmishers) than upgrade/downgrade them to Rhodok Tribesmen or Swadian Recruits. To train a lowly Looter to a Knight might take a while and cost a lot, but it can also be satisfying. There is no benefit in doing so for morale and what the Bandits become is what they become, forever. Therefore, it may be best to just upgrade them to be the best bandit they can be and file under "B" for "bolt magnets" when fighting Rhodoks.

Even the lowliest troops, if used correctly, can turn the tide of a battle or seige. If only through sheer weight of numbers / as fodder.

Manhunters
Manhunters stay close to Looter spawn areas, but they will also attack Sea Raiders and other bandit parties. All troops in the Manhunter line are only equipped with blunt weapons, so if you want to take lots of captives they are perfect for this purpose.

Sword Sisters
Despite slowly evolving from initially vulnerable women, this line results in Sword Sisters, who are extremely powerful fighters on par with the Hired Blades. Refugees and Peasant Women are very difficult to acquire; they can only be hired from a defeated party's prisoners, so for most players this troop line will be very rare. Though the significantly lower wages sword sisters draw compared to their male counterparts can make procuring them very worthwhile from an economical point of view.

If you intend to develop female fighters it is crucial that you keep them out of combat at all costs as they will only be able to hold their own from Camp Huntress onwards (anything lower will be quickly butchered by most other troop types). Once they reach Camp Defender or Sword Sister level they can hold their own against a great many troop types. Sword sisters ride fast coursers and sometimes pack ranged weapons, so they excel against other light and medium cavalry or scattered archers, but densely packed formations of infantry should best be countered with the heavier mounted units such as vaegir and swadian knights and mercenary cavalry.

Sword Sisters assume all roles, that of the (mounted) archer, wielding their crossbows during a siege; that of light cavalry; that of infantry. They do the job quite well in all stakes, but an army composed of nothing but Sword Sisters will need a few Companions or equal or greater numbers against veteran troop types.

While their war-cry is shrill and shrieking, there is not yet a victory-cry for these female warriors, so - when you win - you win in silence.