Koishiteru means: Japanese for “I Love You”, which can be translated to English in a romantic manner. Another option is to say “Daisuki” to someone you’re interested in. You could also say “Aishiteru,” to someone you’re in a serious relationship. When you say “Koishiteru”, it is best to use this expression to someone with whom you would like to share the rest of your lives. (in Community Dictionary, added by Matilde Parra)