Cassie is a human from the Animorphs continuity.

Cassie is an Animorph, capable of becoming any animal quickly and easily. She believes that all of Earth's creatures are on her side in the war against the Yeerk invasion, and alongside them and her friends, she is inspired to become a wolf, using her morph's strength, fierceness and speed to stop the invaders' insidious plot. After breaking into the Yeerks' genetic experimentation lab, she and her friends Marco and Jake have acquired the ability to merge and become the colossal Tri-Rex through the use of an advanced morphing technique.

Fiction

Ask Vector Prime

Vector Prime knew of the Animorphs' exploits, despite them taking place in the Megaverse, outside of the Transformers' multiverse. Ask Vector Prime

Toys

Animorphs

Sadly she got replaced by Kate Hodge when the show changed to Love And Curses.

• Cassie/Wolf (Deluxe, 1999)
  • Accessories: Raker Claws, Yeerk slug

Part of the first wave of Animorphs toys, Cassie transforms from what is evidently meant to be a human into a wolf, as well as a transitional third mode combining features from both. Like many Animorphs toys, she has a pair of wolf claws, the "Raker Claws", that attach to her hands like gloves; unlike the other toys, they are not actually necessary to complete her wolf mode (as it has four paws already), though they can still be used in that mode. Her 'transitional mode' head is spring loaded and revealed when a button is pushed. Like all deluxe class Animorphs she includes a rubberized clear-plastic Yeerk slug.

• More information on Cassie (wolf) at TFU.info

I make Terragator feel inadequate!

• Tri-Rex (Super, 1999)

• Known designers: Tim Bradley and Masakatsu Saito (concept art)

Released in the only combiner set released in the Animorphs toyline, this version of Cassie was available in a Super Class three-pack along with Jake and Marco and could combine into the larger Tri-Rex. Unlike most Animorphs toys, this figure has no normal human mode and only has a 'transitional' dinosaur/human hybrid mode. She transforms into the tail and spine section of Tri-Rex, with the enormous tail becoming a rather unwieldy arm mounted whip. The tail is soft and rubberized as it can be bent but straightens out when released as a simple whipping gimmick. The toy is something of a shellformer and has multiple dino chunks hanging off most of her limbs which hampers her otherwise impressive posability.

• More information on Cassie (Tri-Rex) at TFU.info

Notes

• Cassie holds the dubious honor of having perhaps the least informative bio note ever. It mentions her wolf mode, but beyond that it simply reiterates the Animorphs premise, with no information whatsoever that specifically pertains to Cassie herself.
• In both of her toys, Cassie wears what is probably intended to be green overalls with a yellow shirt underneath. She is the only Animorphs toy that does not change clothing from toy to toy. Even Visser Three changes from red to yellow and black in between his Mega and Ultra toy.
• For Cassie's packaging art, a photograph of the actress that portrayed the character in the Animorphs television series, Nadia Nascimento, was used for both her wolf mold and the Tri-rex toy.