Help finding a mecha anime name?

I was wondering if anyone could help name this anime for me, I've been scouring the internet and I can't find it anywhere.

I haven't seen it in years but what I can remember is-

-It was made at least 10 years ago, maybe longer.

-Two alien races come to Earth at war with one another

- The leader of one of the races I'm quite sure is named "Andromeda"

- The main protagonist ends up finding three mechs with his friends and helps out the above leader's race.

-When the good guys are in trouble, occasionally another mech shows up autonomosly which I'm pretty sure they name "zero".

-The main character ends up able to control the new mech by merging with it.

-A high ranking officer for the good side ends up defecting to the other side to control a mech with the same power as the "zero"

If anyone knows what anime this is please let me know, it's driving me insane!