What kind of character complements Rasaad the best?

Chronicler

Like if I wanted a player character that would pair well with him.

jmerry

The party's big enough that he can fit with any player character you want. Six characters is a lot.

But, when it comes to character balance issues ...
Rasaad, like any other monk, is a pure warrior that leans strongly toward the skirmisher archetype. Fewer hit points than a fighter and no helmet, but he hits hard, moves fast, and can sneak up on enemies with his stealth. You'll want other characters to fill the tanking role, you'll want a thief to handle traps and locks (though it doesn't take much of a thief), and you'll want spellcasters of both the arcane and divine variety.
Monks also get a lot of nice defensive perks. High magic resistance, enough that you can boost it past 100% with the right equipment eventually, a bunch of specific immunities to things like slow, poison, and charm, and even permanent normal weapon immunity at level 20. You can't stand up to high-end physical attackers like fire giants, but you can ignore most things spellcasters throw at you and slaughter the mooks that don't have magic gear. Plus some surprises like mists - did you know that every single mist-type enemy in the game has a weapon that isn't flagged as magical?

Silver_Arrow

As I understood the question it's not about full party but rather player character plus Rasaad only. Imho, monks are pretty weak comparing to most of other classes. I would love it to be otherwise. It's just my opinion, so don't hate me too much for that.

So basically you'd need someone pretty powerful and quite a handyman to accompany Rasaad, or to be more precise - to babysit him while playing with comfort.

If race is not an issue for this playthrough then I would suggest going for a multi-class character just to avoid Rasaad babysitting your player character instead, which is a no-no for a casual play, I think.

You'd have to make a decision if you want to sacrifice on thief skills, divine or arcane powers, otherwise you will be stuck with F/M/T choice as your best handyman.

Anyway, you can pick any multi-class, because, honestly, everything would pair well with Rasaad to compensate for his weaknesses. So just pick something that will bring you most joy either gameplay wise or roleplay wise.

Chronicler

Okay, it seems like I'm gonna have to clarify.

I am playing him with a full party.

But I'm looking for like noteworthy synergies.

Like he can't wear armour, but mages have that armour spell they can cast on him, so that's handy.

And I've heard inquisitors and monks are a really potent mage-busting tag team. The inquisitor dispels all the mages defences and the monk tanks all their attacks with his magic resistance.

Stuff like that. Like if I were specifically planning on being a cool battle couple with rasaad, what class do you think would be best?

Stuff coming at it from a roleplaying angle is also welcome. Weirdly I think the jester works really well for the romance, since pretty much every flirtatious line with him has him playing the straight man to your jokester.

VanDerBerg

And I've heard inquisitors and monks are a really potent mage-busting tag team. The inquisitor dispels all the mages defences and the monk tanks all their attacks with his magic resistance.

You don't even need an inquisitor, you can just absorb (almost) all their spells with MR and low saving throws and use sun soulray and sun soulbeam to wipe the floor with them. Both count as abilities, rather than spells, so go through spell protection and affect creatures who are normally immune to lower-level spells (liches and rakshasas). I am doing a solo run with a sun soul monk and it is a breeze so far. At level 15, he defeated the city gates lich without any problems.