It's never fully explained who MeLaan is, so I'll give you the background here. One thing that kandra do is take Contracts serving mankind in exchange for atium. However, there are other jobs that kandra can do back in the Homeland. One of the more prestigious ones includes the training and instruction of a child kandra.
This can take years and years, as kandra grow very slowly. TenSoon was appointed as a "parent" of a single kandra during his lifetime. (Many of the Fifth Generation have been parents dozens of times, but the Thirds are a rebellious group, and it was only after much consideration—and political pressure in the Homeland—that Thirds were given chances.)
MeLaan, then, is kind of TenSoon's adopted daughter. She has something of a hero-worship crush on him, inspired by his gruff style and adventuresome personality. Her idolizing of him borders on a romantic crush, and this makes TenSoon somewhat uncomfortable.
There you go. Now you can astound your friends with Mistborn background trivia.