On the May 25 episode of JTBC’s “Secret Sister,” Red Velvet’s Seulgi and Sunmi bonded over how well they knew each other.

In a video they filmed for their first mission, Seulgi correctly answered Sunmi’s birthday, but failed to guess her blood type (only getting it right on the fourth, and last, try). But Sunmi knew everything about Seulgi, including her birthday, blood type, shoe size, hometown, shocking Seulgi.

When asked how she knew the details, Sunmi shared that she’d looked up Seulgi’s profile online beforehand. In an interview, Sunmi added, “You’re going to be my little sister, and it’s not right if I don’t know anything about my sister. I have to know things like this to become friendlier with you.”

Seulgi shared her admiration and said, “I couldn’t believe she’d prepared that much for me. It got more comfortable after that.”

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