Baby Shark
I started by extracting the contents of the babyshark.jar file with:
jar -xvf babyshark.jar
Inside the kctf/constants/ directory I found a file called Strings.class. I ran it through an online Java decompiler and got a Strings.java file.
Looking at the decompiled code, I saw several Base64-encoded strings. The last one, _0xflag, stood out. After decoding it, I got the final flag.