Okay, 3, 2, 1, let's jam.
Mystical Child of Light
The Irish Child of Light Cu Chulainn has answered your call, but instead of armed with his spear Gáe Bolg, he comes as a Druid. Using runes, he can preform high level magic. He's an agreeable fellow, provided you aren't a scumbag, but either way he will follow your orders in the end. He has Protection from Arrows, allowing him three perfect dodges, and his Noble Phantasm Wicker Man has good damage, summoning a giant wooden man that scoops up its enemies, placing them inside the cage in his chest and lighting them on fire. He has even technically won a Holy Grail War.
All in all, a good summon.
The Younger Child of Light
But some strange twist of fate, the Irish Child of Light Cu Chulainn has answered your call as well, but instead of coming in as a Druid, or even as his adult self, you summon him in his younger and more spirited days. Like his older self and his Druid self, he has Protection from Arrows, allowing him three perfect dodges from attacks. His demeanor is similar to his older and Druid self as well, but unlike them, he is more hot blooded.
His Noble Phantasm is Gáe Bolg, a cursed spear. Once its name has been called, the cursed spear reverses the nature of causality, to make it so the cause of the "lance being thrust" comes from the effect of the "opponent's heart being pierced" by it. It determines the opponent's fate simply through its use, an always fatal move that pierces the heart with one thrust. Only people with high luck or defenses can hope to survive it. A solid summon.
The Toughest Saint
The holy woman Saint Martha has answered your call, along with the evil dragon she appeased, Tarasque. She is a perfect paragon of a Christian Saint, calm and composed... on the outside. On the inside, she's a rough and tough city girl that defeated a dragon not by prayers like she claims, but by beating it down with her bare fists. However, she will mostly use her staff in battle, and uses it to hit people with, and heal herself, as well as take away the buffs her enemies may place on themselves. She can also fire holy blasts of light... almost like a grenade.
Her Noble Phantasm involves the dragon Tarasque, as she will hit him with her staff and send him barreling into enemies like a tactical nuke. A good summon.
Lancer of the Sun
The Lancer Karna, one of the main characters in the Hindu epic Mahabharata, has answered your call. He was renowned for a spirit so selfless and noble that it shamed the gods themselves. He is actually one of the most powerful known Servants, having armor made of sunlight, a weapon able to fire out nuke level explosions with ease, and unless you know his True Name, all his stats will appear lower to an outsider. On top of it all, it is impossible to lie to him.
The drawback? His immense power will drain his master of so much energy it could kill them. As such, he must be careful of how he fights and how much energy he uses. He is loyal though, and will make sure not to kill you.
His main Noble Phantasm is Vasavi Shakti, which allows him to fly with wings of fire, and blast the enemy with basically the sun. A good summon... but don't get cocky.
The Tengu Samurai
The young samurai Ushiwakamaru has answered your call. That is only her childhood name, however. You may know her better as the samurai Minamoto no Yoshitsune. She is a samurai searching for a master to be loyal to after being betrayed by her beloved brother. Her swordplay has been honed to near perfection, and she is a great tactician. However, she has absolutely no concept of fear or worry, which may land you in more trouble than needed.
She has 4 Noble Phantasms, but only one is known. Dan-No-Ura Hassou Tobi creates eights boats for her to jump across and close the gap between her and her enemy, and slashing them for great damage. All in all, a good summon.
The Manliest Man to ever Man
The Berserker Beowulf has answered your call, and boy are you lucky. The Berserkers are a trap in the Holy Grail War, their mana drain known to kill masters, and their Madness Enchantment making them hard to control. However, Beowulf actually comes with a very low drain, and almost no Madness Enhancment. In fact, Beowulf is noted to actually be very gentle with women, children, and people he's responsible for. Still, this doesn't stop him from being a very powerful servant.
He has two swords: Hrunting and Naegling. Hrunting is a special sword, drinking the blood it draws, becoming bigger and more demonic as time goes on, and automatically makes the best possible attack when swung, something that's really good with the strength of a Berserker behind it. It's also capable of sniffing out blood. However, his swords don't actually make him stronger. They're power limiters.
His Noble Phantasm is Grendel Buster. At the cost of sacrificing his own weapons, he is able to discard Servant limitations entirely and show off his true strength free of all restrictions. The strength of a man who regularly punched dragons and monsters to death. All in all, a very good summon.