He is seen to be a cheeky little otter and he gives Tarik the reflexes of the otter. Lumeo also gives Tarik minor water powers, as seen in Fire and Ice.
He and Tarik are not seen in the Spirit Animals Game.
In the Books
Lumeo's tattoo is first seen when Conor notices it on Tarik's (then just another Greencloak) arm. Later, Tarik calls him up for a demonstration on how to use your animal's powers to your advantage. He is next mentioned when talking about who would take up the Ravens' freedom challenge. He is later seen attacking a mongoose spirit animal. Lumeo is most likely a North American River Otter since Tarik is from Amaya, which resembles both Americas.
In Hunted, Lumeo doesn't really play a role. The only part where he is majorly noticed (or even comes
out in the book) is in Chapter 2. The gang is being ambushed by Conquerers on their way to get the talisman from Rumfuss the boar.
"'Watch out!' [Rollan] shouted. 'It's a trap!'...Two men plunged out of the brush, a fox on their heels. In a decisive move, one seized the bridle of Finn's horse and the other threw himself at Tarik. Lumeo, Tarik's spirit animal, an otter, twisted suddenly out of his dormant form." (Stiefvater, page 46).
Here, Tarik had to summon Lumeo in order to protect them from the ambush. Lumeo plays a small part here in fighting against the ambush: "As Tarik parried [the Conquerers'] blows with astonishing precision, Lumeo pounced on the cat in a chaos of fur and tooth." (Stiefvater, page 48).
Lumeo also has a very strong bond with Tarik, and is willing to help him, as shown here in Hunted: "Conor and Rollan each took one of Tarik's arms [he was wounded] and hauled him up. Lumeo stood by his side, his coat uneven and sodden from the fight in the damp underbrush. His normally playful expression was keen, trying to anticipate what Tarik might need from him. 'It's all right, old friend,' Tarik said to his spirit animal....'Don't worry.'" (Stiefvater, Pages 51-52)
Near the end of the book, Gerathon and General Gar kill Tarik, so Lumeo attempts to avenge him by attacking Gar and in the end kills Gar and his Spirit Animal, Renneg, the Wolverine. Lumeo dies in the process and is buried alongside Tarik by Irtike and Snake Eyes (Mikak)
Lumeo's name came from the word for "light" in Tarik's native language, and was named that because he is the brightest thing in Tarik's life.
Otters are cute. If you have read Harry Potter, you may know that Hermoine Granger's patronus was an otter. So her Spirit Animal would be an otter. Cool, isn't it?