Shadow is the animate, sentient shadow of Di: https://www.weasyl.com/character/56556/dietricha-merriweather-tillens.

Shadow has a threatening appearance, but an ultimately harmless and childlike personality.
She is completely unable to talk (okay, except for a select few rare times) and has to express her feelings and emotions through non-verbal actions and expressions. She is also able to do loud and irritating [through gritted teeth] charming screeches and roars, which sadly, no one is able to understand.

She has a great relationship with Di, but is still on cold feet with Ben (but he's slightly warming up to her).
The rest of the team is okay with her presence, as well. Almost everyone else is afraid of her. [sarcastically] I'm not sure why-- she shows up next to me --because you're such a wonderful, puppy-dog! pats her head

Shadow's physiology is kind of a mystery.
She exists in-between a tangible and intangible plane. Which explains why she can be touched, but why she doesn't weigh anything.
She doesn't seem to have any working organs of any kind, no need to eat, no need to sleep, no need to use the bathro-- holy crap, she is like the EASIEST pet to take care of! I want ten!
She has no genderly defining aspects, so her design is somewhat ambiguous. The only reason she's referred to as a female is because: 1. she looks like Di, who's a female, 2. she doesn't exactly object to it... (and, she's able to object to things, believe... SpongeBob snaps ...me.), 3. I don't exactly like referring to her with they/them and it pronouns...

(I'm not sure if that's offensive or not, I'm not good at judging what people will be upset at. ...well, I assume it'd be a problem on Tumblr at SOME point.)

Appearance (courtesy of the When The Fur Flies Wiki):
Shadow has a solid charcoal black body, that is in the exact shape of Di's, but much more simplistic. She has large, pearl white, pupilless, eyes that are supposed to resemble Di's glasses (because of the bridge in the middle) that occasionally have shines in them, a big, black nose on her long muzzle, a long crinkly-shaped mouth (with a noticeable wrinkle at the end, which is uncommon with my characters) home to a row of sharp, jagged, misshapen white teeth (which she loves to do uncannily creepy smiles with), large squishy cheeks, a large tuft of fur on her head (or something that resembles it, it's unknown if she has fur or not), light black pawpads and a short, fluffy, solid colored tail. She also tends to wear a charcoal black/light black/white version of whatever Di's wearing at the moment. At times, she wears a black collar with a silver loop and a silver tag with a first aid symbol, turned on it's side, to resemble an X (but it's usually when she's not wearing anything else).