Climbs magic stairs that don't let evil people up them by making his energy signature that of Goku's

"I assume you mean whatever technique you attempted to leave with," Pikkon said with a nod. "The answer is yes. The secret entrance has similar protective magicks, since any resident of Hell who found it could simply climb the stairs up. I'm not sure what exactly the spell is, but there is one clue. Carved into the bottom of the staircase are the words: 'The chains which bind the doors of hell are closed within thy soul.'

"Oh, so it's...what exactly?" Goku fumbled.

"Clearly, Kakarott, it's referring to the binding magick. It's connected to the evil within one's heart so only innocent fools like yourself may pass."

"Your interpretation is likely correct," Pikkon continued. "I've traveled that way more than once and have never encountered any difficulties, though, so I have no knowledge of its nature. Regardless, just as the magick of this realm has weakened you and these other villains, it's doubtful you'll be immune."

...

Suddenly, Goku felt a shudder run through his body. His own ki was there in the room with them, somehow detached from himself. How...what…? He and Vegeta realized what was happening in the same moment, spinning around behind them in shock.

"CELL!?" roared Yemma, whipping around. Before he could say another word, an invisible force flung his massive body across the room, slamming him into the wall with a deep rumble. Cell's form rose up behind the mahogany desk, his super perfect aura crackling with lightning.

"You fools!" shouted the android, his eyes wild with crazed excitement. "I overheard your entire plan! After all we've been through together, you still haven't learned that when I say I'm going to do something, it's as good as done! I told you I'd be free of that pit...but still, you fail to comprehend the depth of my cunning!"

Goku burst immediately into his Super Saiyan form, but his eyes were wide in utter amazement. Cell's power was immense, far beyond his own. "But...how did you...how could you make it up the staircase? There's just...no way you could have achieved the inner peace Vegeta did!"

Beside him, Vegeta also exploded into his Super Saiyan state. "Answer him, Cell! How the hell did you get here?"

TAC. Cell landed on the desk with his arms folded across his chest. "Nice choice of words, Vegeta," the bio android chuckled. "Don't forget, Goku: I have your cells. Climbing the staircase is a matter of nothing more than clearing your ki signature of evil, and your ki signature gave me all the purity of heart I could ever need. Even in my imperfect form, my control over ki was enough to fool all of your friends into thinking I was you. In this form, tricking the staircase into accepting your ki again just after you had passed over it was child's play!"

- Bringer of Death Chapter 75