Aska and Luna share the power of the Summon Spirit of Light in Tales of Symphonia. They both must be present at the Tower of Mana in order to make a pact with them. In order to call forward Aska, you need to go to the Linkite Tree and have wind playing through the nuts into a nice sound. Sheena Fujibayashi can only summon Luna while she is in Over Limit mode. When she decides to summon Light, she says the following words. "I call upon the light of the heavens! I summon thee, come Luna!" Why she summons Luna and not Aska is because Luna is the pact maker. When Luna is summoned she casts Ray on all enemies on the field.
In Tales of Phantasia Luna acts as the spirit of the moon totally separate from Aska although both games are supposed to be set in the same world.