Man, trying to pin down the key characters in 'Leveling Ragnarok' is a bit of a moving target because the web novel has so many arcs. The core is obviously the protagonist, Siegfried (often just Sig), who starts as your classic underdog in a tower-climbing world but inherits this ancient 'Ragnarok' system that's way more brutal and OP than standard leveling. His childhood friend and eventual party anchor, Elara the shield-maiden, provides the moral compass and keeps him grounded when the Ragnarok power starts messing with his head.
Then you've got the major antagonists that define different phases. There's General Vanir, the cold, calculating guild leader of the top clan who sees Sig as a threat to the established order. Later, the mysterious 'Oracle of Twilight,' Aelia, becomes pivotal—she's not purely an enemy or ally, more a manipulative force with her own agenda tied to the tower's origins. Don't forget the side characters like Brokkr, the cynical blacksmith-artificer who crafts Sig's gear and offers the best sarcastic commentary. The cast expands a lot in the later 'Asgard Recursion' arc, but Sig, Elara, and the shifting role of Aelia are the emotional core through most of it.