Honestly, I don't think Gasper gets nearly enough credit. At the start he's pure comic relief, the scaredy-cat vampire locked in a box. But his arc is basically about learning to control his own power, which is a metaphor for controlling his own identity and emotions. The whole 'Forbidden Balor' thing isn't just a power-up; it's him finally accepting the monstrous part of himself instead of running from it. He stops being just the 'cute trap' side character and becomes someone who can actually fight for his friends. The way he idolizes Issei and then slowly grows into his own person, not just a copy, is handled pretty well for a side character in a series with a huge cast. Sometimes the pacing feels rushed, but the moments where he stands up, especially in the later light novels against formidable foes, hit hard because we've seen him at his most pathetic.
I will say, though, his dynamic with Valerie Tepes adds a layer beyond just the usual harem antics. It's a surprisingly sweet, almost protective connection that grounds his development in something other than battle prowess. That relationship gives him a reason to be brave that feels more personal than just wanting to be like his senpai.