Five Character of the Heroes of Olympus: The Son of Neptune Revealed

Also known as the god of death. This guy controls where souls go after people die. Fun job, huh?

What does he do? Acts as a lieutenant of Pluto (Hades), Thanatos acts as border control to keep souls from crossing between life and death without his permission.

Noteworthy: He's a good looking guy. Who knew the god of death was a looker?

This is not your typical ridiculously skinny half bird half woman. Ella the Harpy is more than meets the eye, and a very useful ally for demigods Percy, Hazel, and Frank.

What does she do? If an encyclopedia could take the form of a woman (or bird), it would be Ella. Any questions from Latin phrases to the definition of photosynthesis, she knows the answer.  
Noteworthy: Don't let her tiny body fool you, she's got a massive appetite - just don't feed her cheese. Ella doesn't like cheese.

Stuffed animals around the world should fear when Octavian is around - meet his newest victim.

What does it do? Act as a willing participant in this young demigod's regular rituals. By sacrificing stuffed animals, like this poor teddy bear - Octavian is able to read the will of the gods - very handy, if gruesome.
Noteworthy: Octavian is a demigod for Percy and friends to watch - if not careful, their fate could resemble that of this pile of fluff.

Reyna and her Hounds:
These dogs are precious in more ways than one. They protect Reyna, a very important demigod who holds a hand in Percy's fate.

What do they do? Aside from sparkle (one is silver, the other gold), these hounds act as Reyna's personal bodyguards.
Noteworthy: These dogs are not just encased in metal on their bodies, they have rubies for eyes. Talk about valuable pets!

Frank Zhang:
Frank, a new ally of Percy, has the last name of Zhang - as in "Master of Bows". With this trusty weapon at his side, this chubby-cheeked boy is primed to become one powerful demigod.

What does it do? This handy piece of weaponry is Frank's go-to tool of choice in battle, with pitch-perfect aim and easy mobility. He can carry it around, just like a backpack.
Noteworthy: A master of weaponry, Frank has a few other tools at his disposal, including a spear and certain flexibility in form that might come in very handy down the line.