Marlene is I-No's electric guitar that she uses as a weapon. I-No uses it in both magical and physical attacks strumming it to produce damaging sound waves or slamming it to her opponents. She also uses it for flight as she rides it like a witch's broom when she dashes in the air. Marlene is also listed among the things I-No likes in her profile.
Visual presentation-wise Marlene is an "almost-direct" replica of the Duesenberg Guitars company's "Starplayer TV" (also known as "Starplayer II") real-life guitar model, albeit made with reinforced materials (such as titanium and space flight-grade duraluminium) to withstand heavy impacts such as when I-No is hitting her opponents, directly smashing her guitar against the ground, etc.