[DragonShade]

As mmm said, yatte is the te form of yaru

te form is used as a fragment to connect with all kind of "postfix"


yakusoku shite kara yatte ageta
kanojo de sonna koto yatteru yatsu irutte

Ageru means I do something for somone
(Ageru, Morau, Kureru, = gold lol )

The sentence means I will do it for you since we have promise
The second one , hope I get it right: There's that kind of guy who does that to her / their girlfriends

Correct me, if I get them wrong ^^