The original lineup of DCUO NPCs were largely based on Jim Lee's style and I personally label this the "Hush Era" because the big deal about that story arc was that we saw Jim Lee's Batman (along with Bat Family and Rogue's Gallery) and Superman in the same comic books. In a sense, this is what Supergirl would look like if she was mixed into that arc. This is also the pre-New 52 design. Thankfully the New 52 costume never made it to DCUO!
Original DCUO Supergirl
Bombshell Supergirl (2015)
The Bombshells designs actually started out as fun illustrations by artist Ant (Aunt) Lucia, which then became variant covers. Out of popularity, the comic books turned them into an alternate universe in a WWII-like setting (Earth 24). And now, they're in a short DCUO episode (#15, Bombshells Paradox & Corrupted Zamaron, 8/12/2015) back when content was being released monthly. Though this design didn't batch replace Supergirl across the main content, it was an alternate universe character that appeared in the Bombshell Paradox raid (and today appears in the House of Legends location). You could also purchase the Legends skin for LPvP/LPvE gameplay.
CW / Melissa Benoist (2016)
In 2016, DC Universe Online and CBS did a cross promotion of the new Supergirl TV series with a new in-game skin. For a short while, the TV Supergirl was the main model for the game, being swapped in as the vendor, ride-along and Legends skin. This design sports Melissa Benoist's visage in a cartoon/anime style along with the darker, realistic costume with carbon fiber texture.
CW Supergirl, Melissa Benoist
Here is some rare footage of this design in the game:
Rebirth + Melissa Benoist (2016)
Aside from the CW Supergirl being a limited time visual swap, there was a directive from DC Comics to not mix live action styles into the game. At this point, the CW version was updated and the Batman v Superman base items (which were scans from movie material) were removed. Some remnants still remained as people who bought the base items still have them and anyone who bought the Legends skin can still run LPvP and LPvE as CW Supergirl.
Rebirth Supergirl Costume on Melissa Benoist
The updated "main" Supergirl in DCUO that's in the quests, vendors, etc. still kept Melissa Benoist's features, but her costume was in the DC Rebirth style, which updates from this point on were leaning towards. This was the design for most of the life of the game to this point.
Rare footage of the BvS posters:
NOTE: While DCUO was not allowed to use "live action" styles, if those styles made their way into the main series comics... like Wonder Woman, it became fair game. The current Wonder Woman has a Nicole Scott style for facial features, but the costume for the most part is the DCEU one because they eventually made that the standard look for the comics in Rebirth.
Rebirth + Milly Alcock (2026)
With the launch of the new Supergirl movie in the new "DCU," DCUO updated their Supergirl design. The current version still adheres to the guidelines of not using a "live action" style in that the costume seam pattern and coloring are still DC Rebirth style, but the model is Milly Alcock, who plays the current cinematic Supergirl. Supergirl joins the adventures of Luzia in the latest Story Arc, Children of Krypton.
Supergirl in Argo
Red Lantern Milly (2026)
Later on in the Episode 53, Children of Krypton: Shadows Over Argo storyline, we see a Red Lantern Supergirl. Since it's in the same episode, the model underneath the costume is still Milly Alcock. While it doesn't appear Supergirl will be a Red Lantern in the movie, one never knows with easter eggs. Maybe the ending has a WTF moment that segways into the Lanterns TV series, meant to be in the same new DCU. There could also be a post credit scene as is common today.
Alyssa Goldenblade and Red Lantern Supergirl in Sunstone Refinery