Everyone is better at different things.
i DONT SEE WHAT MAKES Usagi a good partner.
In my opinion:
Kotaro(all game)is better(he really is good)
I would argue that Korato is worse than Usagi on the ground of he has 2 borgs for most of the game and you get him pretty late game.
He is much better in special mode however.
Tsutomu is better(his tanks rock)
Tanks do rock, but in a different way. They fill a different niche.
You would be crazy to bring a tank team in a battle against 3 opponents. They are not able to dodge and can't soak that much damage. They will get batted around and die early.
Everything has their own place.
Yuji is better(nukes, hello!)
I think that Yugi is actually one of the worst partners to use because his bubbles slow you too and his nukes come when you least expect them. I don't know about you, but I don't like being nuked.
Tama is better(durability, again)
Tama is ok.
He puts all of his eggs into one basket so if the other team is composed of 3 teams or has a lot of heavy ranged hitters he is not the way to go.
Everything has their own place.
Orochi is better(not sure, she's awful balanced, but better)
I am actually not that fond of Orochi.
Why?
Akuma Samurai.
I have to put all of the hefty borgs in the front of my team because all of the little 5hp nicks and scratches that I sustain from getting clipped halfway across the stage really eat away at my borgs.
Also, those nicks and scratches ruin my combos.
Sho is better(have you seen his attacks? He can survive nukes, with most of his borgs)
I don't like Sho.
I know that I will be getting a lot of hate for this, but I don't.
I don't know if it is just my game (because I have noticed that the AI will act a lot differently on different files) but ee tries to juggle all opponents whether I am next to them or not.
I will get hit by his bombs ALL THE TIME.
Friendly fire from him will usually deal more damage to me than the enemies.
Nekobe is better(chainsaw knight, torture for his opponents, and all his other knight borgs)
I am probably also not going to make friends with this comment either, but Chainsaw Knight sucks.
I mean, I love the borgs too, but it is just way too slow and awkward.
Also, I usually pick Nekobe for my partner and I have to say that a lot of the time all of his borgs will die off before the match is done.
I don't know about you, but I don't really like taking on 2-3 other borgs out of nowhere if I am not picking "none" for my ally.
Oh yeah, and Mana is better.(HEALING? HELLO?)
Once again, I am probably going to upset people with this, but I think Mana is terrible.
This time, because she is almost too good.
Oh sure, she has the same problem as Nekobe where she will run out of borgs before the match is done. But I don't like her because my borgs don't die.
I know that this is weird to read. But I want my borgs to die.
I don't have such a hard time with the game that I can't afford to lose anyone, but I don't want to go through a match only having used 2-3 borgs.
This is because it deprives the back half of my team of exp.
Not every borg on your team gets exp, only borgs that you actually used in a match get exp.
I like exp.
Especially because I play multiplayer a lot. I don't train nurse borgs because they do not fit my play style, so I can't make a Mana team mate for an AI ally.
And even if I did train them I only have 1 of each borg in my box so that is a whopping 3 nurses.
Also, in multiplayer the support borgs get taken out super fast, so if I did have a team of 8 nurses I would be all alone in a few minutes staring down the other player and their AI ally.
Also more leveled borgs means more options when making a team.
All of this adds up to me thinking that Mana is a very poor ally because she doesn't help me out in the big picture.
Usagi sucks, in my opinion.
Has little durability, with her borgs
smacks me around, and ruins MY range combo, when she's trying to hit me with quickness
Freezes me. Often.
Only good then is when she hits me with quickness, without ruining my range combo.
The only partner actually WORSE that Usagi, is Kitsune, and i do believe that's obvious.(MAGNET ROBOTS!!!!!!!!!!!)
You are entitled to your own opinion.
We all know that in this response I have certainly flexed my own.
I also do not seem to have any of the problems you are having.
Usagis borgs of choice may be frail, but she uses ranged borgs so she sits away from the enemies and is not damaged.
I also do not have the problem of her hitting me... ever.
And Usagi is
easily my most chosen ally.
Girl borgs just seem to work well almost everywhere.
When fighting against melee units she can keep them at bay and stay far away from the fray and their powerful attacks.
When fighting other ranged borgs Girl borgs can fight fire with fire.
When fighting annoying planes and flying borgs there is no better choice than a borg with a gun.
When fighting fortresses tanks are fine and all, but they can't dodge fortresses attacks and are expensive. Girl borgs have more survivability and are cheaper (meaning more on a team and so it is less likely that I will be on my own late into the match)
When fighting robots range is nice because, like with melee borgs, their punches hurt.
They work in many other situations, but you see my point.
When fighting a mixed team comprised of the unknown Usagi is a solid choice
if only because she either has the advantage or is at least not at a disadvantage in so many situations.
Also, Magnet Robots can fall in a hole.
They are so lame and annoying.
I agree completely.
Kitsune would be such a cool ally, if only he never used Magnet Robots.
Thank god he doesn't use them in Special Mode.
But i doubt, as Doggy may have just suggested, she put it in such a style. A girl of her age would probably just shove it up there 
Like i would..
