On the topic of KOS-MOS, looking back at the Xenosaga box arts, I could see her being the 5th DLC as the DLC fighter background color could match with a green-ish hue [similar to Piranha Plant, and due to a video posted on twitter a few weeks ago by someone inspecting the Fighters page of the official SSBU site, and revealing that behind the fifth dlc spot (which has a black rectangle with a question mark in the center), by inspecting the page and removing the rectangle, the background color for that spot revealed to be a green similar to Piranha Plant's]. Can't remember who exactly was it that posted it, but the process was fairly simple.
After deleting the aforementioned line, you will notice that the rectangle next to terry now has a background color similar (if not the same) as Piranha Plant. The color may be a placeholder, OR it may be the actual background color to be used for the 5th DLC fighter. In another thread I mentioned that the color would fit well with Rayman being possibly Fighter #5, but again, KOS-MOS could fit that color as well.