So, the Marth in Awakening is really Lucina, Chrom's daughter, isn't she?
She must've traveled back in time via a time portal with your avatar.
Your avatar probably lost memory along the way and landed someplace random, being separated from her.
Marth (Lucina) probably chose to come out! She entered the past with those Risen to aid Chrom in his time of need.
No wonder Marth can predict the future, she has already been in it!!
She knows everything that's going to happen, and she's there to alter the past.
Also, she learned her skill with the blade "from her father" who turned out to be Chrom! It was a father-daughter duel, all this time.
I can't figure out how your avatar has to do with this, but I think I have the basic story now.
Tell me if I'm right or wrong. But don't spoil it if I'm wrong.