Despite trying and trying, I can’t figure out who Legionnaire plays well with.*
Who should buddy up with Legionnaire, and why?
Edited to add: the only constraint is that his pal needs to be a retail-released model. I’m sure he would LOVE Defender Extreme, but until I have one in my hands… 
*odds are that I have been unsuccessful in figuring out his gaming companion because I have limited myself to models I own. I’m going to guess his bestest bud is somebody I don’t own.
You pretty much nailed it by identifying Defender Extreme, but I get that not having an official model is a bit of a drawback. If you do have the ability to do any modeling or basic kit bashing on your own though, he’s one of the easier models to make a legal conversion of.
I enjoyed playing him with General Hondo to really crank out the ranged attack potential. It is fun having a Rocket Ape get Spotter on a bunch of targets to get the most out of Multifire, but I don’t think it is a particularly good list outside of just being two pretty solid monsters that want to shoot stuff. Defender X or Gakura are both good choices that let you lean even harder into the unit game by making MR Tanks extremely accurate and providing high-accuracy blasts anywhere on the table with Command Matrix or Improvised Weapon.
If you want to get really nasty, you can look at Earth Knight as a partner. The pair was developed by Maren, and it doesn’t play MonPoc like any other list I’ve seen. It leverages Earth Knight’s Defender special rule along with MR Tanks natively high defense to build a board state that is extraordinarily difficult to interact with while chipping away at monsters with unit damage and Legionnaire’s blast attack. The impact is that your opponent is stuck spending a disproportionately high amount of resources going after sturdy screens, Def 9 Earth Knight, and Reactive Shield Legionnaire while you get to plink away with multiple accuracy buffs and a constant stream of power dice from Righteous Fury. Once your opponent does manage to engage, they then have to deal with Earth Knight’s incredible damage output and Legionnaire’s annoying ability to dance around the map, slowly winning the game through efficiently trading damage.
I had the same thought about Hondo (who I don’t own), but also thought “I don’t see the appeal beyond ‘shoot absolutely everything.’” 
Earth Knight was actually my next choice, though sadly mine is nowhere near ready. (What I would give for a Green Fury logo decal for the shield…)
Out of curiosity: what was the rest of that Legionnaire + Earth Knight list?
Every now and then it’s interesting to get external and potentially radically different input.