what is twitch's background?

Ekko sorta implies Twitch is his old pet rat. I think it's safe to assume his old origin is still canon - a sewer rat mutated by the magical, life-altering chemicals that seep into the Zaunite sewer system.

As for his life, we know he's a friend of Zac's, probably because Zac is made of the sludge that Twitch so vehemently worships.

In his original lore he was a sewer rat that got mutated through the chemicals in the zaunite sewers. Though it is currently unknow if this is still canon or not,so we'll have to wait until his lore gets updated.