Susanne of course!

After school, you find yourself in the gym, the pen concealed in the pocket of Milla's leggings. You watch Susanne from afar, her dedication to her sport evident in every practiced movement. You know that she'll head to the locker room soon, and that's where you'll make your move.

Once Susanne leaves the gym floor, you follow at a safe distance. She enters the locker room, heading straight for the showers. The sound of running water fills the room as you stand outside, waiting for the right moment.

As Susanne showers, you prepare yourself. You click the pen five times, ready to act. When the water stops, you know you only have a moment before she comes out. You move quickly, stepping into the locker room.

"Susi, can you come here for a second?" you say, using Milla's voice to lure her out. She peers around the shower curtain, trusting her best friend's presence.

As she steps out, you apologize internally for the deceit and press the pen's knob firmly. Susanne begins to deflate, her form collapsing into a skin suit on the wet floor. You catch her before she crumples completely, laying her gently down.

Now, you must transition quickly. You slip out of Milla's skin, carefully peeling it away and stepping out of it. The sensation of leaving the body that you've grown accustomed to is jarring, leaving you momentarily disoriented and indeed, as you realized, naked.

You stand there for a split second, your own body exposed, before reaching for Susanne's skin. As you align yourself with the suit, the process is familiar yet distinctly different from before. Susanne's physique is unique, her memories and experiences foreign to you. The headache comes, sharp as a needle, but you're prepared for it this time, welcoming the rush of new memories—Susanne's memories—that flood your mind.

Now in Susanne's skin, you feel her strength, her athletic prowess, and the burden of her secret. You dress quickly in her clothes, the fit different from Milla's but no less comfortable.

Finally, you take Milla's deflated form and repeat the process you've become so adept at. With another press of the pen, she begins to fill out, returning to life. You guide her as she regains her bearings, ensuring she's sitting and stable. You use the pen's power to suggest she fell and hit her head, giving her a plausible reason for the gap in her memory.

With Milla now awake and Susanne's skin suit securely on, you leave the locker room, ready to explore the life of your best friend's crush. As you walk through the halls, you can't help but wonder what secrets you'll uncover and how this new perspective will affect your relationship with Tim and the rest of your classmates.