I've been battling with my two starter pets-- they're the first two in my stable, Voltaic and Demeter. I have them both attuned to Earth. Originally, Demeter was attuned to Earth and Voltaic was attuned to Fire, but I noticed that Voltaic was losing health in battles much faster, so I attuned him to Earth too. Their stats are mostly similar, but Voltaic loses health points like a leaking bucket.

The biggest difference in their stats is their base health: Voltaic's is 883, Demeter's is 2445. They've been training together from a low level, and they're the same level now. Voltaic had a head-start since he was my first pet and I started battling with just him.

How did Demeter's base health get so much higher than Voltaic's, and is there any way I can get his base health up?

Earth isn't really a great element to compose a team with- it's a Tank element primarily, and has a chance to inflict Taunt on opponents with its basic elemental attack, forcing the enemies to attack the pet that inflicted it.

The difference in HP, though, is because Demeter has an HP prof point, while Voltaic has a Str prof point instead.

Thanks! Could you point me to some resources on good team builds, if you have a minute to do so?

This might help:

https://www.sylestia.com/forums/?thread=45988