Perhaps if he hadn't regained the memories of their past life, he would've already fled and left tending to her to someone else, but he feels responsible. Unaware of Sailor Venus having already awakened, Mamoru assumes that he's the only one who remembers their past lives. Mercury and Mars don't seem to have a clue, and neither does Luna, who ought to have recognized him. She caught him sneaking around with Serenity as often as Venus and Kunzite did.

Luna at least seems to clue in to Usagi's bad drunken directions and bounds ahead, meowing at them in attempt to get him to steer her in the right direction. It works, and Mamoru prides himself on not blurting out that he knows Luna can talk and instead just playing around with her faux meowing. And while the Mautian (Mauling??) walks to the front door and sits on the mat, waiting for him to follow, he instead sweeps Usagi into his arms and leaps up to her window in a single bound.

How he knew her room was on the second story is beyond him, but he knows, somehow, this is her room. And that she always forgets to latch her window, so it will slide open easily. Enough for him to step inside with her still in his arms.

"I didn't want you to get in trouble for coming home drunk," he explains, setting her back on her feet.