Leanne was a Hogwarts student and a friend of Katie Bell. Leanne went to Hogsmeade with Katie in October 1996 and the two of them spent some time in the Three Broomsticks. Leanne and Katie were walking back to Hogwarts when Katie touched the cursed opal necklace and was nearly killed by the curse (HBP12).
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Notes
We don't know much about Leanne except for the fact that she's friends with Katie Bell. However, we can make a fairly good guess that she's in the same year as Katie, since it seems likely that Katie would be hanging around with kids more or less her own age. Katie is a Seventh Year in HBP. We know that Leanne is not in Harry's year, so if she's NOT also a Seventh Year she would be a Fifth Year like Ginny. While it's certainly not impossible that a Seventh and Fifth Year would be friends, it seems far less likely than for Leanne to be a Seventh Year as well. The books also do not reveal Leanne's House.
From the Web
Leanne on the Harry Potter Wiki
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Tags: cursed item