Well. That narrows it down.
*THIIIIIIIIINKS*
Uhm... The only people who come to mind are Maxie but her name begins with a M ... not a L - and Lara ... but she's not changed her name.
*looks through all the members*
J-E-L-L-O?

You made this harder than you probably meant to.
But seriously; J-E-L-L-O's the ONLY person I can find that could probably even begin to fit your clues.

XD
- Is a Prefect.
(The clue is in the colour.
)
- Even though they've not been online for AGES now.
- Has a pretty high post count.