The adaptations flesh her out differently. In the games, she's pretty.. Generic.
I usually play as May, as I see her as the canon Hoenn protagonist and because I prefer Brendan in his NPC role. I've played as Brendan a few times, and May comes off as weird.
Brendan has character development. He goes from being a cocky kid to learning his place.
He has a personality, and is rather Type B tsundere at that.
He has more goals, he wishes to be a gym leader.
I'm pretty sure he has a line that she doesn't; the Rayquaza one in Emerald.
The first time you meet him is a character defining moment. He comes off as arrogant and a bit judgmental. For quite a few meetings afterwards, he seems to think of you as being lower then him in terms of skill.
May.. May is just generically sweet. She's nervous and embarrassed towards you at first, but a few lines later and that fades. She lacks any serious character development. She just asks as the Team Mom, worrying about you for no real reason.
DP had Dawn, while not being as fleshed out as Lucas, with more of a personality then May did.