To know the truth of Serdic's name, do not kill Marie. Then you keep playing until your main character is Altrius.
Beat the game once and save your game to unlock the harder difficulty mode.
Hidden stage 9 ~Harbour and Priates~: Don't let any of the civilians die in stage 8. You'll get Cotton after you finish stage 9.
Hidden stage 19 ~Underground prison~: Talk to Yumiluna with Serdic on stage 18. Have Shino talk to Izuna to recruit her. Talk to Alberich with Serdic if you want him on your team later on.
You must not kill Marie in order to find Hidden Stages!
They're many stages, if you kill Marie you will become Divine Emperor, way you didnt kill you will become another version which you doesnt use Holy Blade but Spanta
Don't allow any of the townspeople to die in Stage 8.
You can recruit Alberich by talking to Yumiluna with Serdic in the Uther stage, then talking to Alberich in the prison cell with Serdic, then talking to Alberich again in another stage.
It's a long process, but I think it's worth it, getting him.