In the last part of Episode 4 when Lindo exorcises the vampires on the roof, the two white parts on his eyes appear to glow red (earlier in the episode he says that vampire eyes glow red) then turn back to white. He then states that he will protect Ritsuka and "you'll see". From afar, the vampire known as Jek, who is hidden behind a wall, appears to recognize Lindo. He wonders why Lindo would want to protect Ritsuka. It also possible that he also wonders why Lindo killed those vampires since they are possibly the same.
In Episode 5, his eyes turn all red and when Mage sees them, he seems to know what he is and he doesn't like it. Since vampires and devils appear to be enemies, this is probable the reason.
Almost all but confirmed Episode 7 and 8 where he grows black claws as well.
- Confirmed. In Episode 9, the Exorcist Society reveals to Ritsuka that Lindo is in fact, a dhampir (which is the result of his mother, Marta, having a relationship with a vampire).