same, though given Koko is a "Gladiator", it makes no sense for Koko to use a "practical" weapon, he uses weapons with style points (in 5e, there are just martial and simple weapons, though the book also says you can just reflavor and reskin the existing weapons into other weapons using their stats, hence why flails and clubs are flavored to be "gauntlets")
anyways, in regards of the spiked shell, the 5e explanation of the shell ability (your AC is a flat 17, despite your Dex, and you can't wear armor)
though seeing Koko has the "gladiator background" which allows him to have a "costume" I do garnish "spikes" on his shell, less for actual practical weaponry, and more of thematic flair.
being a gladiator, its less about being practical, and more of being practical, but with flair (hence why the gladiator background is just a variant of being an "entertainer" background, you're really fighting to entertain others, like from reading how gladiator fights went out in real life, it's literally just like the WWE, except the violence is legit, for all I know, Koko might not be as tough as he makes himself to be. and "Killer turtle" might just be an act he kinda holds up)
unless the wizard actually casts "fire shield" on Koko, his spikes are harmless, or too impractical to use, unless the situation specifically calls for it (body slam), if the wizard cast "mage armor" (which by the book, says you can choose to change your AC to "13 + dex" when wearing mage armor instead of what you actually use) on Koko, it would actually do more harm since his dex is complete garbage
AC and armor is not an "addition", armor is just a stat change, unless its a shield, then its a +2
