I want to reward feat or skill to certain character after they finished a certain quest.

How can I do that ?
Which function I should call ?

Skill: ApplyEffectToObject(DURATION_TYPE_PERMANENT, EffectSkillIncrease(SKILL_HEAL, 1), GetFirstPC());

changing SKILL_ to whatever you want, the number to however much you want to raise it, and GetFirstPC() to whatever character-getting method you want.

Feat: a little more complicated. In theory, as far as I know, you could add a creature hide item to the character (using slot INVENTORY_SLOT_CARMOUR), and give it the item property Bonus Feat. There's no script, other than that, which can add feats. I haven't tested this method though, so it may not work.

From my understanding, the creature hide is certainly the way to go. I'm pretty sure that's how they do it with the subraces in NWN1.

The most straight forward way though would just to give them an item (or add the property to an item they already have).