Shifters are pretty horrifying, that girl on all fours is a brainwashed child soldier, she walks like that because her shifter is quadrupedal so she got used to it, inhereting a shifter power is done by eating the former host after being turned into a mindless "pure titan" with a spinal fluid injection, they all die 13 years afterwards. Eren became a shifter after eating his dad and has like 5 years left.
The island with the 3 walls they all think is the last bastion of humanity is actually a "penal colony" for their race (only they can turn into titans) that hasn't been wiped out yet because of an alleged dead man's switch, the mindless "pure titans" oaming outside are mostly people that got exiled and turned into titans as punishment by the rest of the world which views their race as subhuman devils (shifter soldiers help the humans outside fight the island partially because of brainwashing and partially to earn a chance for their families to become honorary humans and be granted minimal human rigths).
