(Note: The sizes are from different points in their lives.)

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Here is a comparison showing both Gwen & Khris.
Gwen, wearing the green shirt, is her current look, while Khris, wearing the dark purple shirt, is his current look.
The younger Gwen is based on how she was when she was founded by a research couple that lived near a shoreline and studied the local wildlife there
The young teen version of Khris (shirt with logo) is how he looks a year after he was introduced.
Here, Khris is still shorter than Gwen, as she's already nearly the max size for the females of the species, while he still has some growing to do, as males are taller (about a foot more)
Young Khris was adopted by the same people, although he was a little older than when Gwen was founded.
Teen Khris's feathers have a more "slicked" back appearance, showing that he is a bit more martured while he was out traveling. (Of course, there is still a fun side to him)

Side note:
In the image of younger Gwen, she is meant to have 4 greenhead feathers, but ended up with 3. This will be fixed later on
Want to point out that, despite Khris having a teen look, comics that feature him will continue to have his younger look.
Also, despite how their species looks, they are not technically penguins; they are more along the lines of unusual mammals with penguin-like features. (Much like how the platypus looks like it's a strange furry bird with its duck-like bill). But for the sake of the tags, they will be listed as "penguins"
Currently working on their species background lore some more.