Disclaimer: Honda et al belong to Kazuki Takahashi. As in, not me.
There's liking someone, and then there's really liking someone, and then there's liking someone enough to give up your soul for them twice. So when Mai comes back to Domino, Honda doesn't get in the way. Instead he smiles a lot. Widely. Showing plenty of teeth.
It's not that he doesn't understand what Jounouchi sees in her. A nice rack, for one thing. Money, for another. And she can be nice when she wants to be. Top it all off with the fact that she and Jounouchi are pretty evenly matched in Duel Monsters, and there's a lot to like about Kujaku Mai. He gets that.
But what he doesn't get is how that outweighs everything else. How she treated them all like dirt until Jounouchi finally beat her. How she took out her issues on Jounouchi by trying to feed his soul to a crazed Atlantean's pet monster. Oh yeah, but she was sorry, wasn't she? So sorry she took off without a word once the whole mess was over, and then came waltzing back one day as if a smile, an apology, and a swing of her hips would make it all better.
And it did. Because somehow, despite what a bitch she was when they first met her, despite the whole giving-up-his-soul-for-her-twice thing, despite Doma, despite everything, Jounouchi still likes her. Maybe even loves her. And there's no point in saying anything about it, because if it came down to a choice between the best friend and the girlfriend, it's not too clear which one Jounouchi would choose. Which is why Honda keeps smiling, and doesn't get in the way.
Even though his jaw really fucking hurts.