Rudi is a type of alien that is mildly inspired by insect design (and Invader Zim). He shares some aspects with many insects, and one of the obvious shows of the correlation is the pupa stage.

Despite seemingly being physically mature, he is actually a late bloomer. Slow growing physically, so he would lack many capabilities his actual fully grown species would have. He is what they consider as a Youth.

Youth -
Frail exoskeleton.
Rare possibility of wings, they are only six inches at the largest and are incapable of flight. Very fragile.
Heavily depends on other means of offence instead of strength.
Has only a 30-40 foot radius of technology control.
Mildly mentally unstable (emotionally or thoughtfully)
Antisocial
Spines are restricted to forearms
Spines are frail and don't do any harm
Smarter than adults
Black exoskeleton that hides internal organs.
Nomadic.

Youth are vary frail and incapable of much physical battle. As a result they depend on hiding and outsmarting their enemies.

This instinct to hide and win in clever ways to attack carry on in the Youth, and are hard to fight.

When metamorphosis begins (around their teens to [what we could consider] early 'adult'hood) the shell is already growing on the spines, and maybe sometimes on any joints. This causes paralysis and restricts movement. On occasions the shell will even attach to something, keeping the Youth restrained to one area.

The youth will often move to an area they believe is safer, or stay if they are trapped or are already in a secure place. The shell will continue to grow until it has fully covered the Youth.

The process lasts only seven hours, so there is a strict time frame for any that are in the open or wish to move. When fully cacooned, they tend to look like statues. Being frozen in place, the shell showing their outline. The shell will continue to grow for a day after covering the pupa, afterwards they will begin to change. It takes only a week.

Thwn they become an adult.

Adult -
Spines grow on back, forearms, lower legs, and chest.
Spines are poisonous.
Have wings that are capable of flight. Three meters at max length. As durable as plastic.
Possibilty of having a stinger.
Durable exoskeleton.
Can affect technology up to 200-450 feet away.
Can somewhat affect minds, just enough to speak with thoughts and to create weak emotions.
Exoskeleton can be any color, and are slightly transparent.
Rare possibility of getting extra limbs.
Mentally sound.
Heavily depends on frightening or surprising enemies before fighting (suddenly showing wings, puts extreme focus into invoking fear via mental capabilities, etc.)
Antenna have barbs on them.
Very social

Since the two are drastically different in mental and physical ways, it is easy to assume the two as different species. If the adult does retain the black coloring, it is easier to make the connection.

Sometimes there is a mutation that they don't turn it no adults at all, and it is fairly common. 26% of the population had this oddity and remained youths until death.

There is also a surprisingly very uncommon possibility of being a late bloomer, only 7% had this. There is a higher chance of mutations for the late bloomers: they are more likely to get the extra limbs, the stinger, and a chance of being stronger with the mind abilities. Late bloomers don't have a specific deadline, although it commonly happens shortly after young 'adult'hood.