Expressions (phrases, clauses, etc.), Godan verb with 'su' ending
to be profoundly compatible; to be inseparable; to be so close as to reveal to each other the bottom of one's heart
彼とは学生時代から肝胆相照らす仲だよ。He and I have been inseparable friends since our student days.
Also written 肝胆相照す【かんたんあいてらす】