For the game, dunno, he likes to give titjobs to Jaina and Eric, also likes to tease Derik with his cleavage(and works well).
Krowly's clearly bisexual though, and enjoys sexually pleasing his lovers. Nothing about that, in any way shape or form, means he's trans.
Hell, Eric would probably be doing the same if he had big tits of his own.
What categorize a trans? Isn't the same thing?
The same as futa?
Depends.
The word futanari literally means "New Half", and is itself in reference to shemales/trans-women. The term also includes highly-feminine hermaphrodites (women with a dick/dick and balls at the same time as vagina), which is basically the stereotypical standard for a futa character.
Or surgery is needed for that confirmation?
IRL, no, that's typically a part of the actual transitioning from man to woman and vice-versa. All that's actually needed is someone born one gender, and only actually feels like themselves when living and dressing as the other. Surgery is only part of it.
At the same time, simply dressing in the clothes of the opposite gender doesn't make someone trans either.
Even tho is through potions, for me he still categorized as trans
Except, as you point out, he's still he. A man with big boobs isn't a trans woman.
so it's fair to have futa/trans protagonist tag, he can't be categorized as futa, but can as trans.
He "can" be categorised as both, but the reality is that he's neither.