One Straw Hat Pirate who deliberately did not unlock Haki abilities.

SUMMARY

While the Straw Hat Pirates have reached their height, Nico Robin had the potential and opportunity to unlock Haki, but she chooses not to.

Robin learned a lot while training with the Revolutionary Army, but she couldn’t access Haki’s skills, which makes no sense.
If Robin had Haki’s power mixed with her Devil Fruit, she could have become one of One Piece’s most powerful characters.

Although the Straw Hat Pirates have earned the title of Emperor of the Sea, they still have a long way to go before they can finally reach the One Piece treasure. Along the way, they are going to meet some dangerous opponents who will try to stop them from achieving their goals, just like it is happening right now on Egghead Island.
Nico Robin from One Piece | Toei Animation
Therefore, their current power level is not strong enough to face the danger, and they might soon need a new power-up. While most characters have reached their peak, there is one character who had the potential and the opportunity to unlock Haki, but for some reason, she doesn’t. This character is none other than Nico Robin, who spent her time with the Revolutionary Army.

Robin learned a lot while training with the Revolutionary Army, but she could not unlock Haki’s abilities, which makes no sense. If Robin had the power of Haki combined with her Devil Fruit, she could have been one of the strongest characters in the history of One Piece.

Nico Robin’s Journey During Time Skip

During the Sabaody Archipelago arc, when Batholomew Kuma sent every member of the Straw Hat crew to a different place, Nico Robin was sent to Tequila Wolf, a country in the East Blue that was under the tyranny of the World Government. However, the Revolutionary Army saved the region and freed the people from the clutches of the World Government.

It was then that Nico Robin was taken to Baltigo, the former base of the Revolutionary Army. There, Nico Robin met Monkey D. Dragon, Sabo, Koala, and the rest of the members of the army. Nico Robin stayed with them for two years and learned a lot about the history of the world and the ways of the Revolutionary Army.
Nico Robin With The Revolutionary Army | Toei Animation
Not only that, she also perfected her fighting and Devil Fruit abilities. She learned Fish-Man Karate and also harnessed the true powers of her Devil Fruit. She could increase the size of the limbs she can create and also fly using her multiple hands. All these new skills helped her and the Straw Hat crew a lot.

Nico Robin’s Inability to Unlock Haki Makes No Sense

Initially, the Revolutionary Army was going to help Nico Robin reconnect with the Straw Hat Pirates. However, when Luffy declared that the group would meet after 2 years, Nico Robin decided to stay with the Revolutionary Army and train with them to get strong enough to defend herself and her crew from danger.
Nico Robin from One Piece | Toei Animation
While she learned a lot of new skills, like the Fish-Man Karate, she, for some reason, did not learn how to access Haki. Considering the fact that Haki is one of the strongest abilities in One Piece, it should have been her first priority to learn the technique, as it would have made a perfect combination with her Devil Fruit.

As of right now, Robin has not used Haki even once, and if she had unlocked the ability, she would have already used it. It could have saved her and the Straw Hat crew from a lot of trouble and even aided them in their escape from Egghead Island.