Illarion is a Russian male name, the Russian form of Ancient Roman Hilarius meaning “cheerful happy” via Ancient Greek hilarosἱλᾰρός (cheerful, merry, glad)< hilaos (gracious; kind) which ultimately derives from a PIE root word.
Tatum comes from an English surname, a variant spelling of Tatham, originating as a locational name for someone who came from a place called Tatum. The second part of the name comes from Old English hām meaning “homestead, village, estate” while the first part is made up of Old English personal name Tata, itself of unknown meaning. It’s possible that it may be derived from Old Norse teitr (glad, cheerful, merry).
Serena is a Late Roman given name, the feminine form of Serenuswhich derives from Latin serenus meaning “clear, tranquil, serene”, originally used to refer to clear, calm weather, and, figuratively, “cheerful, glad, joyous”. Though the origin of the word is uncertain it has been linked to PIE root word *ksero- (dry).
Serena is also an Italian and Catalan surname originating from the given name.
Serena could also be used as a Japanese female name (also spelled Serina) with a variety of meanings depending on the kanji used:
seri 芹 “parsley” + rei 怜 “wise” + na 菜 “greens; vegetables” (芹怜菜);
sei 汐 “eventide; tide; salt water” + rei 玲 “wise” + na 菜 “greens; vegetables” (汐玲菜);
se 世 “world, society; generation” + rei 礼 “manners, etiquette; bow, salute” + na 奈 “Nara, what?” (世礼奈);
se 瀬 “rapids, current” + rei 令 “order, command, decree” + na 奈 “Nara, what?” (瀬令奈);
se 瀬 “rapids, current” + ren 恋 “attach; love; romance” + na 奈 “Nara, what?” (瀬恋奈)
Written in hiragana it’s せれな. Serena is also written as セレナ in katakana.
Origin: uncertain, possibly Proto-Indo-European; Japanese
Meaning: from a Late Roman name “clear, tranquil, calm”; as a Japanese name, it has a variety of meanings depending on the kanji used in Japanese
Usage: English, Late Roman, French, Spanish, Italian, Catalan