Last Blade 2-in-1 Play as Musashi
With the controls at the default setting, highlight a character and press Triangle(6), X(3), Triangle(4) at the selection screen.
Alternate costumes
With the controls at the default setting, highlight a character and press Circle at the selection screen to choose their second costume colors. Highlight a character and press Start at the selection screen to choose their third costume colors.
The image featured at the top of this post is ©The Legend of Heroes: Trails of Cold Steel III key art / Original