Unmarked spoilers are ahead, per Spoilers Off rule.
When Izumi is emotionally devastated over being told that he has no talent for the manga industry, Ryouma personally goes to Saotome with Izumi's manuscript and has him review it. Even better, he wrote an inspirational message to Izumi encouraging him to keep pursuing his dream. He even drew Lala-Lulu herself telling Izumi that she's rooting for him.
Ryouma's support in general for Izumi's dream of becoming a manga artist is very sweet, despite the fact he tends to not understand manga all that much.
The mere fact that Rei stayed awake for two nights awaiting a message or a call from Izumi. He knew that he must have hurt Izumi's confidence when he told him that his manga was awful and he wanted to apologize to him. The moment that Izumi arrived home, he hugged him and shed tears of joy.
Rei's realization of Izumi's feelings for Ryouma, and him telling the boy that he wouldn't object to it, believing that Ryouma is a good man despite him saying it'd be a boy-boy couple. He then internally admits the main reason is because of how popular Ryouma is and that would be great for Izumi's career, but his actions in helping him with emotional support about it prove the contrary.
Rei has yet another moment in the tenth episode when he tracks Izumi's location. Rather than tell Shougo where Izumi was, he responds by distracting Shougo by asking him to come over for a drink after work. He knows very well how overprotective Izumi's brother is and he was giving Izumi and Ryouma a chance to be alone together.
Shougo's habit of putting Izumi, and to an extent Rei, over his career as a singer. He constantly drops everything to come and help them, even going as far as to fly in from another country to see them and track down rare merchandise for his brother.
