Let's have our first two growing aliens, then, shall we?
Y'La is a genetically engineered human-like lifeform designed by an advanced alien species to interact with, blend in to, and gather information about humanity. Unfortunately, it seems that much of their information about human psychology and biology was... Inaccurate. Y'La stands at a mighty seven and a half feet tall, with muscles and curves to match her size. Extremely pale skinned, with green hair and golden eyes, Y'La would stand out even if she wasn't so tall. Due to her creators' lacking knowledge of human psychology, however, Y'La sees nothing out of the ordinary about her appearance, and her personality is excessively cheerful and bubbly - in fact, her first task upon planetfall is to find a friend, in order to gather information more quickly.
All of this would be bad enough on its own, but Y'La's creators apparently didn't understand that humans stop growing. At all. Ever. And thanks to her engineered personality, Y'La won't ever think a single thing is wrong, even when her toes could cover entire countries. Neither will she understand why the little humans run away from her, instead of wanting to play or talk. Buckle up, Y'La's going to cause a bit of a bumpy ride!
Mika is an alien shapeshifter of an almost-infinite age, being one of the oldest beings in existence. Exploring thousands of planets, Mika is no stranger to interstellar travel and alien civilisations, but she is currently trapped on Earth, as her spacecraft was destroyed many years ago. Being fearful of humanity's potential for violence and frustrated about being cut off from space, Mika has attempted to wait until spaceflight is achieved by humans, but something's gone wrong; her shape shifting abilities have gone berserk, trapping her in the body of a young teenager undergoing an eternal puberty. While good-natured and, to a degree, shy, Mika's evergrowing body will soon make it very hard for her to hide from humans...