You can upgrade characters in various ways, starting with their stats and gears, which will boost their power overall. Let's take a look at them.

You can upgrade their weapons (by heading to the Fatima tab - Weapons) and then start leveling them up with Gold to boost their Attack. More weapons will be unlocked as you collect weapon fragments.

You can upgrade their skills (by heading to the Fatima tab - Skills) and then leveling the skills up with Gold. More skills will be unlocked the more you rank up your Fatima (androids).

You can upgrade their stats power (by heading to the Fatima tab - Plugins) and adding the right plugins which give various stats (tap on the plugin to see what bonuses it gives and select the ones which are "best" - marked as red- for each character).

You can add Insignias to your characters, which also boost their stats (by heading to the Fatima tab - Insignia), but these are of two kinds: General (fit all characters) and Exclusive (character-locked). The Exclusive ones are better designer for the specific Fatima it fits.

You can further boost their stats by adding Gears (head on to the Fatima tab -> Gears) and select the Gear which is better fitted for the specific Fatima. These Gears' batteries will run out, so I suggest that you either use Battery Charge to repair them and fill up their battery life or equip others (or use Toolkits to remove them after use).