A (А): A bit colored like English O. B (Б): A drill. V (В): Equivalent of D. G (Г): Equivalent of C, except he dies in episode К to save И from Й's accent. D (Д): Equivalent of E. He is a car. Ye/Je (Е): Main antagonist, and equivalent of F. He spells СОДЕЙСТВИЕ, which is assistance in Russian. Yo/Jo (Ё): Equivalent of G. ZH (Ж): She plays her own role. She tries to help the good letters stop Ye. Z (З): Basically just Ye's hiding spot. He only gets beaten up by Ye after the good letters don't notice Ye. I (И): 1st equivalent of N. She quietly saves Й by using Г's dead body as a stool, and her skirt as a lasso. When killing Ye, she uses Й's accent mark and fires it at Ye. J (Й): 2nd equivalent of N. She looks like J and Y combined, as Й sounds like Y, but actually is J. K (К): Equivalent of L, and is the first to transform. L (Л): He is very boring, but he does have a gem. M (М): Equivalent of M. N (Н): Equivalent of P, and is the last to transform. O (О): I don't really have an explanation for him. P (П): A glass table. R (Р): A scared-looking english P combined with R. She also doesn't get kidnapped. S (С): A rattlesnake. T (Т) Equivalent of I, except he doesn't get used as a gun. U (У): I don't have an explanation for him either. F (Ф): I don't have an explanation for him either. H (Х): Equivalent of X, but he doesn't own the asterisk gem. He is X combined with H. C (Ц): Equivalent of V. CH (Ч): Equivalent of Y. SH (Ш): Equivalent of W, but is so powerful that Ye even uses her power to hypnotize other letters to join his side. SHCH (Щ): A fence that Ye uses as a gate to his cave. Hard sign (Ъ): Equivalent of Q, except he's 5 times slower than Q. Y (Ы): I don't have an explanation for him either. Soft sign (Ь): Ы's sister who is confused about everything that is happening. E (Э): A girl who joins the heroes. She spells "эхо" (echo) to alert the heroes that Ye is nearby. Yu/Ju (Ю): I don't have an explanation for him either. Ya/Ja (Я): Equivalent of Y (at the end of episode Y). He isn't the planet, as the Russian letters live on the bottom of Z (English).
Credit 2 Every1, especially @On_Beyond_Zebra139 for Ye's base.