Serph could recognise the distinctive silhouette of Agni anywhere. But why is Heat here of all places? And... does Heat still want to kill him?

It's not important; he had told Heat they were comrades, and he meant it.

Serph doesn't call his name. Instead, Serph simply enters combat beside him as Varuna, unsheathing a blade to cut off the needle fused arm from a red Unknown grasping for Heat.