[Prophesized Hero] servants:
Mash Kyrielight, Artoria, Artoria lily, Artoria Archer, Castoria, Lalter, Gilgamesh, Casgil, Atalante, Nyalter, Cu Chulainn, Cascu, Proto Cu, Georgios, Ushiwakamaru, Summer Ushiwakamaru, Heracles, Lancelot, Saberlot, Sakata Kintoki, Rider Kintoki, Hektor, Diarmuid, Saber Diarmuid, Arjuna, Karna, Santa Karna, Fionn Mac Cumhaill, Beowulf, Astolfo, Edmond Dantes, Rama, Sanzang, Sitonai, Minamoto no Raikou, Tristan, Gawain, Saint Martha, Ruler Martha, William Tell, Siegfried, Archer of Shinjuku, Yan Qing, Proto Arthur, Penthesilia, Musashi, Nezha, Achilles, Okita Alter, Sigurd, Bradamante, Asvathaman, Asclepius, Jason, Paris, Spishtar, MHX, MHXX, Mandricardo, Odysseus, Dioscuri, Caeneus, Watanabe no tsuna, Percival, Rider Caeneus, Jiang Ziya, Charlemagne, Roland, Kriemhild, Don Quixote, Super Orion
Flavor wise, it's a simple matter of Chiron using his divine wisdom to both defend himself (Mind's Eye True) and to employ a much more powerful version of the Coaching skill (WHY OUT OF ALL THE MENTOR SERVANTS IS THE ONLY ONE WITH A DEDICATED MENTOR SKILL MAID ALTER?!?!) and reduce an ally's cooldowns sharply while also lowering Chiron's normally quite high star gather to nothing so his allies can more easily crit (which also internally synergizes well with his AOE crit up and star bomb second skill) and using his medical knowledge to increase the effectiveness of all healing received by allies, though he's reliant on said allies to actually provide the healing.
Taking is a step further, Chiron cannot use his cooldown reduction on himself unless he is solo, and if the servant to receive his mentoring is an actual mythological hero just like the ones he trained, then Chiron's own legend is amplified and he gains several more effects. He shields his allies (apply target focus), is more in his comfort zone teaching them (reduce own cooldowns by 1), and fulfills his divine duty as a mentor of heroes thereby strengthen the constellation he became as a reward for his service to said heroes (NP strength up).
Balancing wise it also gets pretty crazy.
Out of all freaking EIGHT of these effects, less than half of them would scale with level, and one of them is Healing Up on a servant with no healing skills so really it's more like two and a half. In exchange, those remaining skills would scale IMMENSELY high, and the reduced cooldown on its own is partially worth leveling for given the potential uptime on the skills and the increased frequency of the non-scaling utility effects. The defense buff would go from a 20% max to a maximum of 35%, the potential NP Strength buff would be a maximum of 50%, and the at its best the HP Recovery Up would be a whopping 100%, effectively turning it into something that means under ideal conditions as long as Chiron is on the field all healing the team receives is doubled. The defense buff in particular is of note because it means that not only does the evade synergize well with the potential target focus, but it also means even against (the nowadays VERY frequent) Ignore Invincible, Chiron would still be able to properly tank hits for allies. The NP Strength up is a one time buff and Chiron generally lacks any personal stims, meaning even if Chiron can get good refund you'll be using that for the utility more than damage the second time. It's still high enough however that between Chiron's damage rank up and supereffective against Earth bonuses, that first NP can easily deal meaningful damage
