Tensei Brave full screen is one keystroke: press Alt+Enter, and the options screen carries a full screen entry as well.
Going full screen
Alt+Enter toggles full screen while the game is running. There is also a dedicated entry for it in the options screen, which is the safer route if the game is running in a window that is hard to reach.
Fixing a broken window layout
If the window layout comes out wrong — for example after the resolution was changed while the game was running — the options screen has a ウィンドウ初期化 (Window Initialize) entry that resets the window layout.
If a main-story conversation scene does not display at all, the reset is bigger: go back to the title screen and reset to the default settings from the options screen there.
Driving the menus with the keyboard
The game asks for a headphone volume setting at launch, and mouse control is unreliable — the pointer can vanish and the sliders are awkward to drag — so the keyboard bindings are the practical way to drive the menus: Z or Enter to confirm, X or ESC for settings, and Ctrl to skip. Two more keys belong to the battle screen, ログ[L] for the log and エフェクトON[E] to toggle effects.
Those same keys carry into a fight, where they sit on the bottom row next to the command buttons. The combat guide lists the full row and what each command does.