[quote=@hatakekuro]
I was thinking of going as Rama, from the Hindu Epic Ramayana.
[/quote]

Oh, the Ramayana characters are totally fine, but as Fiddle said, Rama is a character in Fate/Grand Order. 

The other Ramayana characters and Mahabharata characters (save Karna/Arjuna/Sita) are totally on the table, though. Albeit, Ravana's probably ineligible now given some stuff Rama's said in Grand Order.