Method to Acquire - You have to visit the left house in the northeast or neither house in the northeast on Chapter 9 and she'll start with you on Chapter 11. If you visit the right house in the northeast then she'll join you at the end of Chapter 11 and will do Auguries for you but you won't be able to deploy her.
Damage - N/A
Critical - While she doesn't have any critical, each support with Percival increases his critical by 5, his dodge by 5, and his accuracy by 2.
Speed - Lalum is an exceptionally fast unit who will max out speed almost every game. Thus, almost no enemy will be able to attack her twice.
Dodge - Lalum's dodge should be around 64 at level 20, so if you put her in a forest or fortress, she'll almost never be hit.
Growth Rates
HP - 70%
Strength - 10%
Skill - 5%
Speed - 70%
Defense - 20%
Resistence - 30%
Luck - 80%
Average Level 20 Dancer Lalum
HP - 27
Strength - 3
Skill - 3
Speed - 20
Defense - 6
Resistence - 10
Luck - 24
Total - 93
Supports
Echidna - 20/30/40 turns
Garret - 59/60/80 turns
Ojay - 59/60/80 turns
Geese - 59/60/80 turns
Roy - 59/60/80 turns
Douglas - 10/20/27 turns
Percival - 30/30/40 turns
Rating - 7/10. She's better then Ninian because she can support with Percival to make him unstoppable and her dance takes much less time so it doesn't annoy you to death. Of course, there is nothing like Filla's Might or Nini's Grace in this game, but I don't see that as sufficient reason to deploy a dancer. Her sprite on the map also looks like she's holding daggers.