Rakszan

Rakszan was a Necron Lord of the Khansu dynasty that got a short story from the 5th edition Necron codex. He was the first, if not one of the first, of his dynasty to willingly upgrade to a robot body. Since the rest of his kin didn't dig it, he was upset with them and rewarded for his eagerness by being given his own serving legions to bring his own dynasty to heel. After murdering most of them, he dragged the ones still alive to the biotransference machines to turn them into Necrons.

At first he was OK with this, since he thought it was the best way for his people to live. Unfortunately for him, the C'tan were pissed off at his dynasty for being a pain in the ass against turning into robots, so they sent him on suicide missions or forced him to fight in the vanguard. This caused the remnants of the Khansu to slowly die away, which pissed off Rakszan a lot, so when The Silent King rebelled, Rakszan fought as hard as his little robot heart could to get revenge for his brothers and sisters. After the War in Heaven, Rakszan's dynasty was gone. Everyone was either destroyed completely or stolen and reprogrammed by other Necron dynasties. Since Rakszan was still incredibly butthurt at himself and the C'tan for fucking him over, he roams around making sure every C'tan shard is trapped in a tesseract labyrinth forever.

You might say he's like a grimdark Ash Robot Ketchum.