This form of Grillby was made by Dr. Terminal Gaster in his lab. The purpose of his creation was to see if Dr. Terminal Gaster could make a living monster from the fusion of souls. Gamby has a soul from 3 different fused souls: Classic! Grillby, UnderFell Gaster, and UnderSwap Sans. His body is made from fire magic and nuclear waste, so he must stay in a glass box at all times.
The only one who is allowed near him without a radiation suit is Outertesty, another test subject of Dr. Terminal's.
please go to ask-dr-terminal-gaster to learn more
Gamby MC Radioactive looks extremely similar to Grillby. The only difference is his color scheme. He now has a predominately blue flame and green suit/bow-tie.
He's quite cheery most the time, although he doesn't talk much, but if something wakes him up, he's pissed and screeches loudly, although not enough to shatter his glass box.