先輩のこと好きすぎて死にそう
The girl tells her boyfriend/senpai this after they kiss, etc
He hugs her and replies this to her:
これが素か...こっわ... (可愛すぎてびっくりした!)
What does 素 here exactly mean?
See 素 on jisho:
素【す】
Noun which may take the genitive case particle 'no'
2. plain; unadorned; undecorated; unadulterated; au naturel
素【す】 refers to a plain state of something as opposed to its decorated, modified or processed state. For example, 素のフライドポテト is a French fry without even any salt added, and 素の発言 is a straightforward statement without any joke, sarcasm or euphemism involved.
In your context, the senpai seems surprised at her attitude, thinking, "This is her/your true nature!" This implies that she usually presents a different appearance externally.
(But... did he really say this to her? While feeling a little anxious because someone is "too cute" is understandable, saying こっわ directly to someone in a romantic situation would usually cause a misunderstanding.)
I think it best translates to unbridled.