2D sprite in 3D game

So I believe this is called 2.5D though I may be wrong, I want to add a 2D character into a 3D game, like from a sideview, the only example I can think of is Paper Mario for the N64 and Wii, where it’s 3D and Sideview but Mario is 2D, an example of the Sideview I want is like Mortal Kimbat, but my question is how do I put a 2D character in a 3D game

All games made with Unity are “3D”. 2D sprites are just quads or flat polygons that face the camera.
So you do it just the same way you would any other asset.

In the screenshot below the probe is a 2d sprite. But it’s floating over a 3d terrain.

You’ll want to use the Sprite Renderer on your character. Sprites are ‘2D’ and that is what you will see. However to move around in 3D space you’ll need some 3D physics. Such as a Rigidbody and likely a Capsule Collider.

2.5D is usually when you have 3D characters moving around on a 2D plane. So the renderer is 3D while the Physics are 2D.
Not sure what you would call 2D renderer with 3D Physics, but besides Paper Mario, Xenogear also did something similar.
If you want 2D characters moving on a 2D plane but with 3D background then you will need 2D renderers and Physics for the characters, but 3D renderers and 2D physics on everything else. Skullgirls would be an example of this.

You can put anything anywhere you want, just remember how you want it to be seen (rendered) and how you want it to act (physics).

Hope this helps.