Examples:
Hakushaku Cain Series - Riff often mentions that he wouldn't be able to live without Cain.
Spoiler (highlight to view)
Actually, he had attempted suicide before he met him.
Vinland Saga - the purpose of Thorfinn's life for a long time is revenge.
Spoiler (highlight to view)
And when Askeladd finally dies, but not by Thorfinn's hand, Thorfinn is really shocked and then becomes apathetic and indifferent.
Please no ecchi, hentai, smut, GL.