Grown Regulus
Regulus lives and watches over Harry
Regulus amputated his left arm at the elbow to get rid of the Dark mark and fake his death.
Metal, gears and magic prosthetic.
Hunted horcruxes for over 10 years.
Found necklace and ring and replaced with copies.
Started watching Harry, the order and death eaters after Voldemort came back at the end of 4th.
Cast a patronus to save Harry from the dementors summer before 5th year. Keeping him from being expelled and put on trial.
Wears cloak and mask to keep his identity hidden.
Joins the battle at the ministry to protect Harry and Sirius. Saves Sirius from Bella.
Gets away before he can be identified.
Dumbledore finds the fake ring and realises someone else is hunting for the horcruxes. He does not get cursed and therefore does not ask Snape to kill him in Dracos place.
Snape manages to twist the unbreakable vow so he does not have to finish the job, just protect and save Draco.
Regulus having watched death eaters frequent Borgins follows them through the vanishing cabinet to hogwarts, destroying it behind h to prevent more death eaters coming through.
Having developed an enchantment to sense voldemorts soul pieces he discovers by accident Ravenclaw's diadem and replaces it with a replica.
He learns that Dumbledore is not at the school, and fears he might be trying to retrieve a horcrux. He goes to the cave to check and finds D and Harry there just as the inferi attack. He protects them with fire from the shore as Harry gets Dumbledore back across in the boat.
He is angry at Dumbledore for bringing Harry. He does not reveal his identity. He spells Dumbledore asleep to keep him from recognising him. Levitating him back out of the cave he casts fiendfyre at the lake before closing the entrance to be rid of the inferi once and for all.
He apparates Harry and D to Hogsmead and accios Rosmertas brooms. Levitating D beside him they fly to the Astronomy tower. He knows it is an obvious trap and is ready.
Telling Harry to stay under the cloak and not reveal himself.
He shields Dumbledore behind himself.
He is ready and blocks Dracos disarm.
They speak at wandpoint. He tells Draco he knows it feels like he has no choice now, but that he does. Tells him he is more like his mother, he is good.
As Draco is distracted and uncertain, Regulus binds and gags him with rope and holds him in front of him like a shield as the other death eaters come up the stairs. He whispers in Dracos ear to trust him.
He recognises Yaxley and the Carrows and guesses at Greyback, telling them so.
They talk and he threatens and intimidates until Snape arrives.
Speaking to him by name he tells them to leave or he will kill Draco, which the others may not care about, but Snape as his Godfather and friend of his mother does. His vow forces him to do whatever to protect Draco.
He promises Draco will stay alive but be prisoner to the order and not able to leave the school. He spells a cuff on his wrist that will prevent him from leaving the Hogwarts wards.
At the overwhelming resistance, lack of easy opportunity and lack of support from Snape, the Death Eaters retreat.
Regulus revives Dumbledore and let's Snape treat him with a subtle threat over Draco, but let's Draco go.
At some point he gets Kreacher to nab Narcissa from Malfoy Manor without being seen and puts the same cuff on her.
Harry has remained cloak throughout.
Regulus takes the Malfoys wands and follows after the Death Eaters to make sure they leave and do minimal harm. Harry follows, taking off the cloak when he is out of sight of the Malfoys and Snape.
Rabastan secretly protected baby Neville when LeStranges and Crouch attacked Longbottoms?
RabastanxRegulus?