Stealth Monkey

At first you might think: there's already a Ninja Monkey, and ninjas are known for being especially shady, so is there any need for a Stealth Monkey? Well, let's look at reality a bit and see: the Ninja Monkey in BTD5 is actually not related to being shady or stealthy at all, it's just a monkey that uses ninja weapons and the cloning skill. This monkey, however, is truly stealthy, enough to make all the attacking bloons ignore her and thus will never be targetted, but she (another female monkey!) can still be damaged from splash damage (explosions) and piercing attacks (Laser Cannons), and she can also see camo bloons without upgrades since they're all the stealthy kind. She uses 2 daggers for combat, using them to stab the bloons... quite quickly! As a downside, however, because of the daggers she has a really short range. Oh and she also specializes in optical magic. (wait, MAGIC?!)

Price: 600$ on Medium

Stalker
Change her default attack mode to "Stalk". When stalking, you can designate a target, and she can move freely across the track, chasing and stabbing that target until it's dead or escaped. If no target is designated, she'll automatically stalk the strongest bloon in her range (yes, the range circle's still there). If the attack mode is changed, she'll return to her own position.

Price: 400$ on Medium

Optical Beam
Sometimes shoots a transparent beam like a flash from her eyes. This beam can pop bloons by 1 layer and stun them. It doesn't have infinite range though, but the range is still pretty far. The fire rate is very slow, though.

Price: 1800$ on Medium

Massacre
She's now equipped with some Eversharp daggers so she can just use the daggers to sweep through everything when she's moving, in other words, damaging every bloons on her way while she is "stalking".

Price: 2500$ on Medium

Cursed Daggers
These daggers are not really cursed... more like heavily-modified! The daggers are now dipped in a special chemical that will be injected in a bloon when the dagger's stabbing it. This chemical will slow the bloon down to half, damage it over time and finally explodes after 5 seconds! This chemical even works for MOAB-Class Bloons, but it only works when she's not stalking or when she's stabbing the stalking target, because the physical contact with the "sweeped" bloons (in the Massacre upgrade) are too quick, not deep enough for the chemical to inject.

Price: 4200$ on Medium

Concealment:
Gives the "stealth" status to every towers in her range. If she's stalking, then only the towers that are CURRENTLY in her range circle are effected, which means, if a tower is current in her range circle, it'll be affected, but when she moves away and that tower is not in her range any more, the effect will lose and it can be targetted again. This leads to a very unstable ability when stalking.

Price: 600$ on Medium

Hidden Surroundings:
Greatly increases her range, and also give every towers in her range camo detection. The same rules in the Concealment upgrade apply.

Price: 1100$ on Medium

Attention Idol:
She's now assisted by one of her friends, an apprentice idol monkey that can be ordered to go to a certain point on the track and draws attention, making all the bloons in her HUGE range moves toward her, though they won't attack her (because she's an idol for everyone, after all). If no point is designated, she'll go to the entrance of the track. The effect will lose for bloons that has been in her range for 7 seconds, these bloons will go back to their old way and proceeds to the exit (this is to avoid gameskip). If a bloon is in 2 Attention Idols' range, it'll move toward the nearest idol.

Price: 2000$ on Medium

Eye of God
Special Ability: Gives every towers "stealth" status and camo detection, and shoots a Ray of Doom at the same time. Lasts for 20 seconds.

Price: 10700$ on Medium

Trivia

 * This is a reference to Tsubomi Kido from Kagerou Project
 * The Attention Idol is a reference to Kisaragi Momo from the same series
 * I may make an Idol Monkey in the future... it won't be a reference to the same character, though.
 * The Optical Beam upgrade is a reference to Kozakura Mary, also from the same series
 * If you have all the possible upgrades, it can create a quite tactical use of her (much like the Dartling Gun), not just let her standing there and pop bloons automatically.