LogFAQs > #973306571

You get starlight from the gacha when you either get a dupe of a character you have, or a light cone.

Saving up for a 5* light cone is the most efficient option (it's worth 30 rail passes which is a steal - 5*s normally take 60-80 pulls), but it'll take forever. Buying a character is good value if you don't have that character, and the two characters change every month. Buying special rail passes is worst value but if it lets you hit pity on a limited character you really want it can be good. And never spend any currency on normal rail passes.