This is quite a dilemma you’ve encountered.
I’m currently GMing a very high-XP, combat-heavy game, and I have one character in particular that is extraordinarily powerful to the point of one-shotting nemeses and substantial rivals, putting her well above the other characters, even though they are quite competent combatants themselves.
As a GM, I balance by using lots of smaller enemies so she can’t take out as many in one hit, or by giving enemies counters to her abilities.
That doesn’t help you much as a player though, so let’s start with that.
Have you talked to this player about your concerns? What about the GM, or the other players? Do they share your concerns? One ideal solution from a social standpoint would be if you could work out how to improve the narrative aspects of the other PC, and tone down the min-maxing/overpoweredness. Whether that’s rolling up a new character or simply rebuilding this one will depend. This is just an ideal, however, and only works under ideal circumstances. If the others are not on board, or the player resists those kinds of requests, it is a difficult thing to press without hurt feelings.
As for what you can do, perhaps as a Diplomat your goal should be to personally avoid combat as much as possible. You aren’t a fighter, and your skills and weapons bear that out. Invest in talents like Don’t Shoot! from Charmer, Consider Your Options from Republic Representative, or Diplomatic Immunity from Senator that make it harder to target you, and take Force powers like Misdirect or perhaps Alter to protect yourself and others.
You can also take something like Heal/Harm, which would allow you to serve a valuable role in keeping friendlies alive during or after combat (and it’s a good story hook for a Diplomat, like for an aid mission or the like).
There isn’t much you can do to make yourself a contributing combatant, so I would recommend accepting that and putting an emphasis on your character specifically not being a fighter.
Now, you talk as if this character is a particular outlier among the entire party, so I’ll take your word on that. Consider if you didn’t have him around, what’s your power level when compared to the rest of the party? If you’re close, then maybe he’s the problem. If you’re still very outclassed, then you probably need to emphasize being a non-combatant anyway.
If combat is so much of the game that you think you wouldn’t get a chance to do much if you are mostly just hiding while the bolts are flying, then consider if you built the wrong character for the game. If you think not, consider asking the GM for more non-combat encounters. If the GM is primarily providing combat, the other player may think he has simply built a character to fit the game.
But the bottom line is to talk to the rest of your table and find out if they have the same concerns. I would recommend doing this privately on an individual basis first so you don’t put anyone on the hot seat. It’s very uncomfortable to show up to a game and out of the blue have an argument about if you built your character wrong, or be called to take sides in such an argument (and it might make you look like the “bad guy”). Check to make sure that it isn’t just a problem for you before talking to the other player.