Darth Vader's true identity as Anakin Skywalker was a highly guarded secret, known only to a select few individuals including Palpatine, Obi-Wan Kenobi, Luke, Han, Leia, R2-D2, C-3PO, Master Yoda, Bail Organa, Ahsoka Tano, Darth Maul, Grand Moff Tarkin, Grand Admiral Thrawn, Jocasta Nu, Ferren Barr, Verla, Reva, ...
Though it sounds like he thinks Anakin is dead, he could also be viewing it as Obi-Wan does, where he believes Anakin died when Darth Vader was created. There's a hint that Owen knows Anakin was Darth Vader by the time of A New Hope.
This shows that Rex not only knew about Anakin and Padmé's relationship, but they trusted him to keep their secret. Despite Rex's efforts, that episode of Star Wars: The Clone Wars showed that Obi-Wan already knew about Anakin and Padmé.
The last time Rex and Anakin saw each other was Anakin promoting Rex to commander so he could lead the 332nd Company to lay a siege on Mandalore before the latter rushed to go rescue Chancellor Palpatine from Count Dooku and General Grievous.
Why Captain REX Is Way More IMPORTANT Than You Remember! | Ahsoka Series Explained
Was Rex loyal to Anakin?
Rex was 100 percent loyal to Anakin and was his truest friend out in the field. Anakin had to hide so much of himself from other Jedi, but he could be completely open with Rex, and he knew that Rex would never betray him.
And while his warnings don't work, at the very least, it shows that Maul became aware of Anakin and what he would soon become. With the rise of the Empire, it's likely Maul caught on to Vader's existence pretty quickly.
Return of the Jedi establishes that Yoda was aware that Anakin Skywalker had fallen to the dark side and become Darth Vader. When questioned by Luke Skywalker, he confided that Anakin was Darth Vader. Later, Obi-Wan Kenobi's Force ghost confirmed that Anakin had indeed fallen to the dark side and become Darth Vader.
R2-D2 was present when Padmé gave birth to Leia and Luke, so he knew Luke was Anakin's son – but what he didn't know was that Anakin became Darth Vader. In addition to that, Anakin's fall was surely a painful event in R2-D2's life, one that he definitely wouldn't like to remember or even talk about.
At first, Ahsoka has no idea that Anakin Skywalker and Darth Vader are one and the same. She discovers the truth in Star Wars Rebels when she encounters the Sith Lord in a space battle. What she senses is so shocking to her that she passes out.
But before the battle for Coruscant, they had to split up and Rex went with Ahsoka to Mandalore, and Anakin went to Coruscant. After that, Anakin and Rex never saw each other again.
However, Ventress underestimated Rex's mental strength—Rex contacted Skywalker, but addressed him as his first name, "Anakin," instead of his usual "General Skywalker", which warned the Jedi of something amiss. Ventress left to confront the Jedi herself, while the battle droids held the six surviving clones captive.
Whether or not he was the rebel commando named Nik Sant is the subject of debate: However, in the New Canon Rex did serve with Luke, Han and Leia at least once.
Leia's Force abilities and her lack of awareness of her true identity also contributed to Vader's inability to sense their familial connection. Vader's denial of Anakin Skywalker's legacy and his fear of getting hurt again prevented him from realizing the truth about his children until later in the original trilogy.
Darth Vader acts like he doesn't even know C-3PO or R2D2. They were with him since he was a child. He did everything he could to forget his past. Also, his plating had been replaced many times, and protocol droids all look very similar.
Darth Vader didn't learn he had a daughter until Return of the Jedi, when he stood before Emperor Palpatine. The Emperor's power probed Luke's mind, attempting to goad him to fall to the dark side, and he discovered Leia's existence. "Sister," the Emperor taunted.
Unfortunately, Rex and Vader never meet again though. Why didn't Palpatine feel afraid of showing his evil actions, behaviors and ways to Anakin and the Jedi anymore?
Yep. Darth Maul didn't really say or show straight up that he "feared" Darth Vader, but he openly stated his belief that he could not defeat Darth Vader without the help of the Jedi, therefore on his own.
Rex and Ahsoka are very close friends, having met each other at the battle of Christophsis after she was assigned as Anakin's apprentice. He took an immediate liking towards her and later congratulated her when she helped Anakin destroy the Separatists' shield generator.
Rex is a clone trooper of the Grand Army of the Republic, cloned from the Mandalorian bounty hunter Jango Fett, and serves the Galactic Republic under the command of Jedi General Anakin Skywalker and Jedi Commander Ahsoka Tano.
