Citron

@mikepal19  I use Citron often, he is mostly a shield character and not much of an attack character, his job is to protect his team! He hard-counters Imp, because Spin Dash insta-kills him and the stun makes him easier to hit, and stuns his mech for longer. In addition, his primary does splash which counters imp to an extent and he has a shield to protect himself (don't bother with Navel Laser, except with Imp's Mech)!

As for upgrades:

• I must suggest Emergency Exit as a must-have, it costs 1 upgrade point and it allows you to move and sprint faster for a short time after spin dash resolves. It's worth the cost!
• The Health Regeneration and Health Regeneration Delay upgrades, costing a total of 4 points (or 2 points each), with the regen delay being cut in half, and the regeneration rate being doubled.
• Levelling Up, if struggling to use him and want to level him up quicker, only costs 2 upgrade points and means you don't have to use him for as long, if you would prefer not to!
• Quenching Cover, which gains extra ammo for your primary weapon when shielding damage, it costs only 2 upgrade points, and is good if you swap between attacking and shielding frequently.

I suppose it also depends on how you would prefer to play citron (such as offensively, defensively, extra reviving etc.) The upgrades I listed cost 9 upgrade points in total, so you can't use all of them at once!

I hope this helps you!