With this thread, post if you find a trainer who will call you for free items or rematches(and swarms, however I haven't gotten any swarm calls in this game yet so I don't have proof of their existance) so that hopefully we can narrow down the good trainers and delete the bad ones who do nothing but just call for no reason.
(Also, mods, if there's some website out there with this info, feel free to get rid of this if you want, I just think it'd be more fun posting about our discoveries)

Trainers so far:
Youngster Joey(Route 30) - Rematches, Talking about his RATTATA, Gives you Hp Up after the rematch
Gentleman Alfred(Lighthouse) - Rematch
Fisherman Ralph(?)(Silence Bridge) - Rematch

There's a school kid east of Fuschia who will alert you of gym leaders changing locations... I have absolutely no idea what this means. Anyone care to inform me?

Gentleman Alfred in Glitter Lighthouse has offered to rematch me before, as has a random Fisherman on Silence Bridge whose name I don't recall.

The kid east of Fuschia (to my knowledge) calls you to tell you when leaders are available for rematches and when you can obtain their phone numbers. This is useful if you want to obtain all of the leaders' numbers.

All Ace Trainers! Good rematches, they are.

Specifically the ones leading to Victory Road.