In one of the Urusei Yatsura episode, Ten finds a love locket which Lum once gave to Rei when they were together.
The locket contains a photo of the two of them.
Lum gave Rei one half of the locket containing her photo, explaining that if he opens the locket it will "irresistibly attract her" to him, Rei happily accepts the locket, but suddenly he transforms into his tiger form and swallows it, breaking Lum's heart.
Years have passed and it's now time for Rei to take advantage of the locket magic power.