The House star joined the cast earlier this month as a love interest for Ted (Josh Radnor).
"Jennifer Morrison is on the show in more than one episode," Harris told E! Online. "She's a love interest... or is she the villain to Ted's character?"
He added: "It's a contentious relationship, which makes the sex great."
However, Harris refused to reveal too much information, saying: "This year, we're trying to keep things under the radar a little bit, because every time we say that a guest star's on, then we have to say who they are and then by the time you see the show it's not really a surprise.
"Our show is sort of like Lost. The more you reveal, the more you know, and that would give away the secrets."
The new season of How I Met Your Mother begins tonight at 8/7c on CBS.