Close friends, co-stars or a couple? Whatever their relationship is TVXQ's Yunho, now appearing in "The Night Watchmen," has been spending time with his co-star actress Seo Ye Ji.

Sports Seoul reported that the two have progressed from being friends and are already a couple. They quoted a source saying the two are a "cute couple that consult each other about their acting. Because they are watched, they act carefully around each other on set, but on their days off they go on dates."

Another source was quoted saying that the couple has to hide their relationship because they are involved in separate drama love lines and they don't want to cause harm to the drama.

But something must be going on for the couple to have caused so much comment among drama insiders.

However, a representative from Seo Ye Ji's agency denies that the two actors are a couple. She said they are merely friends who look out for each other. They have more of a brother-sister relationship. His agency, SM Entertainment, also said they were merely co-stars.

But then U-Know Yunho has never dated publicly. Earlier this year the idol-actor appeared on the MBC show "Star Gazing" and spoke about his dating life. He said that he was not comfortable about going public with a romantic relationship, especially on TV.

Keeping the person he loves away from public scrutiny is the best way to protect her.

"When women are in a relationship with a celebrity they are envied," he said. "But when they spilt up, women get hurt more than men do."

He also spoke about his dating style on the TV program "Hello."

At that time he said that no matter how confused the public might be about his relationships, the woman in question would know exactly how he felt. His actions would not be open to misinterpretation.

"People think that I seem cold," he said. "When I like someone I go all out for her. If not then, I just treat her like a sister and leave no room for misunderstanding."

In "The Night Watchmen" he plays a coldhearted guard that ladies can't resist.

Seo Ye Ji plays a young noblewoman who wants to marry the crown prince, played by Jung Il Woo.

He previously appeared in the melodrama "Queen of Ambition." Seo Ye Ji began her acting career last year with a role in "Potato Star 2013QR3."