Charm is a core character statistic introduced in Fire Emblem: Three Houses. Like the other core stats, a character's Charm has a chance of increasing when they level up. Among other effects, Charm affects a character's command of Battalions.
Charm affects character's Gambit Mt (by 1 for every 5 points of Charm), Gambit Hit (by 5 per point) and Gambit Avoid (by 5 per point).
Charm is also a vital statistic for acquiring the Dancer class in Three Houses. The class is a reward to the student representing the house that wins the White Heron Cup, a dancing competition between the three houses. The winner is determined by the participants' Charm. Prior to the contest, Byleth is asked to select a student to represent their house, and is allowed one session to train their chosen student in dancing, which will give that student a permanent Charm boost. Byleth's chosen student will win the competition if they have a Charm stat of at least 13.
Charm can also be increased by having Byleth have a Great or Perfect session with a Tea Time, granting a 1 Charm increase to both Byleth and the character they are sharing tea with.