There's a lot of series where the main character's boyfriend is strong and cool.
I mentioned this before, but a great example is Tamahome, the main character's boyfriend from Fushigi Yuugi. Tamahome, who later is called Taka, is very strong. Tamahome is the boyfriend of the main character and was born with a symbol on his forehead. When he meets his girlfriend, he fights really well to protect her. He learned that his destiny was for him to be her boyfriend and protect her. He is very strong in battle and a great fighter. He has a symbol on his forehead which lights up when he is in battle. He is a great boyfriend and character, he is strong both physically and emotionally, he has a sense of humor, and he is very protective of his girlfriend. The manga of Fushigi Yuugi is a lot longer than the anime, 18 volumes. The anime only goes up to volume 13, but there are OVAs which continue the series differently from the manga. When he becomes Taka, later on in the series, he must regain his power. He is the boyfriend and later husband of the main character and while he is very loving and romantic, he is also strong, manly and cool.
Well, if you read the Ayashi no Ceres manga, Tooya is very strong. However, the anime is very short and doesn't cover a lot of the manga, so if you want to see him in action, don't watch the anime. Tooya is the main character's boyfriend/lover and in the manga his strength is shown. He has a weapon that extends from his hand and he discovers it and uses it in battle. He becomes very strong and is involved in a lot of battles. He is distant, mysterious and cool. The manga has a lot of sex and violence, to warn you. The anime, however, cuts out a lot of Tooya's strength and a lot of the scenes of the plot as well. A particular scene in volume 7, where Tooya and his girlfriend are in a hotel room making out and the company sneaks up on Tooya and tries to kill him, Tooya runs out and chases them all, attacking everyone who is sent to kill him brutally, shocking his girlfriend. This scene, as well as many romance and fight scenes, were not animated and omitted from the anime, he is only tough in the manga.
For a cool character, Yasauki, the main character's boyfriend in the manga series Happy Hustle High, is cool, distant, mysterious, quiet, reserved.
Sakuya from Kaikan Phrase is also tough.
Akito from Kodomo no Omocha, while he does fight at times, is both distant and tough and cynical, he later learns karate, but the series doesn't focus that much on it.
Now, everyone may not agree with me and a lot of others don't agree, but I feel that Mamoru Chiba from BSSM is really a strong character. The anime, 200 episodes, the manga 18 volumes, the live action series. I've seen all versions in their entirety and they are all wonderful. Every version is great. Mamoru is the main character's boyfriend, husband, and destined lover. In the past he was the strong Prince Endymion who protected his lover, Princess Serenity with his sword. He was the Prince of Earth. He dies protecting his girlfriend. He gets reincarnated thousands of years later into the present. When he is 6 years old, his family dies in a car crash and he looses all of his memories. When he gets older, he has a recurring dream about a princess telling him to find a silver crystal and if he does he will recover his memories. In the anime, he falls under strong attacks where he has an alternate personality take over and he fights in battle and protects and rescues his true love, but when he wakes up, he doesn't remember any of it. In the manga and live action version, he does it knowingly and remembers everything, but in the beginning of the anime, he doesn't know about his secret and that he has these powers until he has dreams of it and wonders if those dreams are real. He eventually discovers his identity and is able to transform whenever he wants to. He then finds out the identity of the person he rescues and he protects her. He gives his life to protect her and steps in front of her to take an attack. He is injured, but recovers all memories of his past. Shortly after, he is kidnapped by the enemy he is fighting and brainwashed to serve them. He is put under a brainwashing spell which erases all memories of his past and fills him with dark energy to battle the one he loves. He battles his true love and fights her and nearly kills her. He gets healed and later is reborn. He looses his memories again, but a part of him continues to battle. Eventually, he regains his memories after protecting his girlfriend by using his body as a shield and is able to be the boyfriend of the main character. They are boyfriend and girlfriend for a long time during the series. They are a steady couple and dating for a long time in the series. He is both her boyfriend and her protector. At one point, he has a recurring dream that if he doesn't break up with his girlfriend, she will die, so he makes a big decision, but it all ends up turning out well. He travels to the future and meets his future self, King Endymion, and he and his girlfriend find out that they will be married with a child. In his future as King, he protects both the entire planet and his wife and child. He rescues his girlfriend when she is kidnapped and he protects the world with her. They go back in the past and for a large part of the series are dating, boyfriend and girlfriend and a steady couple. He rescues his girlfriend whenever she needs him. When his girlfriend looses all powers, he protects her and his love and power helps his girlfriend gain new powers. He is very mysterious and reserved and a loving boyfriend. He does a lot of gestures for his girlfriend and he risks his life for her daily. They walk arm in arm, hold hands, hug, kiss a lot and go on many dates. They are a serious and committed couple and deeply in love. He does a lot of gestures for his girlfriend. When it is his girlfriend's birthday, they get into a fight because of a misunderstanding. They make up and he buys her the glass slippers she wanted. However, his girlfriend ends up getting attacked. He protects her and rescues her and he transforms. He faces a battle and is frozen during a battle, while he protects his girlfriend. Eventually, he is freed and is able to battle again. He also protects his future daughter that him and his girlfriend will have. When his future daughter has her heart taken out and is dying, he offers her his life support and energy. Later on, when his girlfriend gets hurt protecting him, he takes a knife and stabs the one attacking his girlfriend. He does a lot of great gestures, he takes his girlfriend to the dentist, he buys his girlfriend clothes. He also has a healing power called pshychometry, where he can heal people with his touch and restore them when they are hurt. When someone tries to seduce him, he stays loyal to his girlfriend. Later on, when the earth is being attacked, he starts feeling pains. He is connected to the earth and when he is attacked, the earth is attacked, he tries to battle, but rests in bed because of the intensity of the pains that are entering him. He is shaking because he is in pain and his girlfriend comforts him. In the manga, he has a golden crystal inside of him, a crystal which will save the world, and his girlfriend helps get the crystal out of him. The golden crystal inside of him is very powerful and strong. In the manga, his kisses with his girlfriend helps give his girlfriend strength many times. In the live action version, he is able to transform into his Endymion form and uses a sword in battle to protect his girlfriend. In the live action version, he is able to transform and merge with his past life's form. He later takes in energy, the energy from the enemy in order to save the planet and the energy later takes over him and he must battle his girlfriend. Later on, he is accepted to Harvard and gives his girlfriend something special at the airport. He does something very special for her, I won't spoil it, but he shows that he will be committed to her for life, as, at this time, he has been dating her for years. In the anime, the plane is hijacked and he tries to battle the villian who hijacked the plane to protect his passengers and to make it back to see his girlfriend. He dedicates his battle to his girlfriend, knowing what his fate will be. In the manga, he gets more powers and is able to use "La Smoking Bomber" as an attack, which is strong, while in the anime he uses roses as an attack, in the manga, he receives special powers and in the live action version, he uses a sword. I can go into more detail, but I won't give spoilers. It may look as though I gave spoilers, but if you see the series, you will know that these are not spoilers, the plot goes a lot deeper. He is in a steady relationship with his girlfriend and they are boyfriend and girlfriend for a large part of the series. He is a devoted boyfriend, husband and father. He protects his girlfriend and future wife as well as his future daughter during battles. His girlfriend calls him Mamo-chan when they are dating. He is very cool. Mamoru is very strong, yet a lot of people disagree with me, but in my opinion he is a very strong and powerful character in all three versions of the story.