[Mettaton chuckles, though it is tinged with a bit of sadness.]

Oh, he was a wonderful Monster. Very excitable and caring and so willing to be there for people he cared about.

[There's a wistful tone to his voice as he continues.]

He was here in Wonderland for a time. When I showed up he'd been here for a couple of months. We were becoming friends, slowly, and it was nice. ...Of course Wonderland gives and takes and he's gone now. Sometimes I wonder what things would have been like if he had stayed longer.