Interesado/a
 

when we say somebody is un interesado or a person is una interesada, in the sense that he or she is nice to you because somehow needs a favor or getting something out of you, how can we say that in English?
The first thing that comes to mind is Selfish, is there a more appropriate word?

When someone is nice to you only because they need a favor or because they want something, we can say any of the following things about them.

he's using me
he has an agenda (he's pushing his agenda)
he's a fair-weather friend
he's an opportunist / he's opportunistic
he's self-serving / self-centered / selfish
he's on the move (on the make)
he's only looking out for number one (himself)
he's a scheming individual (a schemer) / shifty individual / shady individual
she's a gold-digger (said of a woman who marries only for money)

I can think of couple of other terms: He's or she's a manipulator. Most specifically you can say, she's or he's flattering me to get what he or she wants.

Insincere ?

Disingenuous is another

Por interés te quiero Andrés...

Además de las aportaciones ya dadas, Oxford nos da:

Actúa solo por interés — he acts purely out of self-interest / in his own interest.

Con matices se podrían usar otros sinónimos:

he is a self-seeking/ self-serving individal

he acts out of self-obsession, self-absorption, self-regard, egocentrism, egotism, egomania, introversion, selfishness...

Y todas esas cosas... ;-)