There are three different types of gameplay involved in Hangyaku no Lelouch: conversations that progress the storyline in which you can use Geass, battles, and traveling through the field map. Though you do vaguely create a character at the beginning of the game (a character simply referred to when called in battle as "P1", though you can give your character a name), this doesn't mean you are restricted to using this character in battles. For example, in the beginning of the game, P1 is introducing themselves to Zero in a conversation. Shortly after this scene, the game skips to the beginning of the canon story, where you're given control of Kallen Stadtfeld. The game itself is self-explanatory, with only the battles holding a bit of change from the traditional RPG.
Battle is done using a Battle Time Sequence system. The character's knightmare mobility determines when the character attacks. The faster the knightmare, the earlier you get a chance to attack ahead of the opponent. Whenever you hurt or kill an opponent, your Drive Gauge will increase. The Drive Gauge can hold up to three levels for every character. Each level gives the character the ability to use a special attack (or Skill).
Hangyaku no Lelouch also uses the DS microphone feature. Press 
or 
to activate the feature, then call a character's name. The maximum characters in a battle are six (three against three). Call a character on your side and they'll be next in line to attack. This is a good way to make advanced combos, but unfortunately isn't good for the Drive Gauge - performing advanced combos with the microphone feature makes the Drive Gauge decrease.