Anjali Mina from Secret Switch (book 0.5). 

Anjali is a chaotic-neutral antagonist who has the ability to switch people's bodies. She does so with the protagonist Reika Junyin and the deuteragonist Tobias Dance, thus launching the story. Anjali becomes Reika's love interest halfway through the plot. Switch is intended to be a non-crucial introduction to the series' theme of soul manipulation.

Anthony Slaughter from Secret Zero (book 1).

Anthony could be considered the narrator of the entire series, being present throughout the entire series with the exception of Switch. He is specifically the sole narrator of Zero although the story revolves around his false love interest, Lucas. This story takes place over the entire Secret Timeline, therefore following all events in later books to some degree.

Derek Grey from Secret Reflections (book 2).

Derek is one of two protagonists of Reflections, the other being his twin, Dylan. Derek could be considered a character foil to Lucas, the deuteragonist of Zero. Derek doesn’t have a love interest for the sole reason that he just hasn’t met the right person. Any potential s/o would be under normal, non-plot worthy circumstances. Reflections focuses on and further expands on the concept of Liches, a Bodist type of necromancer.

Dais from Secret Curiosity (book 5).

Dais is a cat given human form. They’re a very old cat who is terrified of pretty much everything. Dais was given a golem’s body by Anjali as a dying wish to leave the house and see a child Dais had once taken a liking to under very specific conditions. The original name for this story was Curiosity Killed The Cat because the other protagonist, Danny, dug too much into Dais’ background and eventually broke Dais’ conditions.

Blake Novella from Secret Beloved (book 3).

Blake is the other protagonist alongside Rubee. This story actually delves a bit into the background of Terraria, the origins of people’s powers, and why Rubee and Blake seem to be immortal (they are). Blake and Rubee, in their youth, were engaged. As time passed, their romantic relationship turned platonic and both found new partners over the centuries. Blake is a traitorous, regretless man who spends a majority of this story mourning a romantic partner he actually really loved even though said partner’s power is Remembered Reincarnation. 

“I lost my husband this year to an illness.” “Stop telling everyone I’m dead.” “Sometimes I can still hear his voice.”

Sapphyr Mata from Secret Fate (book 4).