Isis is beautiful, sometimes appearing with multicolored wings. What else would you expect from a goddess of magic?
This comes in handy when Sadie's magical powers are just starting to develop: Isis makes these powers stronger, first without Sadie's knowledge, and then with her permission. This is super useful at times, like when mortals can't open portals. Set laughs at Sadie when she proposes to transport the whole red pyramid to a place outside the desert in order to diminish Set's power, saying that mortals can't open portals during the Demon Days.
Sadie's response? "A mortal can't…But a goddess of magic can" (40.12). Oh, snap!
Isis's excellent grasp of magic also helps her become a slippery target when she's pursued—like, say, any of the multiple times Set has come after her and her young son Horus. Whether they run in human form or escape in bird form, Isis usually has a magic trick or two up her sleeve. But will it help her get ahead in the end?
Thoth describes Isis as "deceptive, power-hungry" (23.138), which is not exactly what we'd call flattering. According to Thoth, Isis is the whole reason Set got out of control and turned kind of evil: Isis manipulated Ra into giving his throne to Osiris, Set became jealous of Osiris's power, and they've been fighting ever since. From Isis's perspective, though, why not do anything to help your family? They're your family.
Isis is so smart that sometimes people attribute things to her that she didn't even do. For instance, when Set reels in Zia in order to consume Nephthys's power but finds that she's not in her assumed host, Set shouts at Sadie: "Is this your trickery, Isis?" (37.51). Nope, not this time, but thanks for the thought.
Sadie realizes the depth of Isis's dishonesty when she has a vision of her parents about to open a portal at Cleopatra's Needle. Her mom did this because she contacted Isis while divining the future. Sadie learns this and is stunned—Isis was encouraging her mom to open Cleopatra's Needle, despite the fact that it might kill her. In a way, Isis is responsible for Ruby's death, because she withheld the whole truth.
Once Sadie and Isis fully merge, though, Sadie understands why she acts that way: "I saw her entire history—her early days grasping for power, using tricks and schemes to find the name of Ra. I saw her wedding with Osiris, her hopes and dreams for a new empire. Then I saw those dreams shattered by Set. I felt her anger and bitterness, her fierce pride and protectiveness for her young son, Horus" (39.92).
We know she does it for the good of her loved ones, but yeah, Isis is kind of manipulative. If it were over issues like "eat your vegetables" or "do your homework," maybe it wouldn't be such a big deal, but this is huge, world-changing stuff. We hope it's worth it.