Phineas is not just Ferb's stepbrother, he is also his best friend. Fеrb always helps Phineas with their daily activities. If it wasn't for Phineas' creativity and Ferb's knack for inventing things, their daily activities would never happen.
An infrequent running gag in the series is that Phineas accidentally causes Ferb bodily harm as a result of the former's obliviousness.
The two have known each other for almost their whole lives, and spend every second of the day together. Perhaps the greatest example of the duo's everlasting friendship is when Phineas states that he couldn't ever ask for a better brother than him.
Due to the fact that Ferb rarely speaks to Candace at all, not much is known of Ferb and Candace's relationship. Besides, unlike Phineas, Ferb hardly ever expresses his respect and love verbally to Candace. But it can be made clear that he loves Candace as a brother loves a sister and is willing to help her when she needs it.
Just like his stepbrother, Phineas, Ferb doesn't display any exasperation despite Candace's unsupportive attitude. Actually, both dialogue-logic interaction and physical interaction between them on screen are much less than the ones between Phineas and Candace. Whenever Candace threatens that she will tell mom about their, Ferb usually simply sits or stands beside Phineas, staring at her without other reactions, despite the fact that he also suffers from some of her obsessive behaviors. As it's often Phineas that face Candace directly and reply their thoughts "instead of" him.
Though Ferb's silent most of the time, he is not actually as shy as others would assume. He isn't afraid to tell his personal thoughts towards Candace when he thinks it necessary. Also, Ferb's wry wit is often exposed in the conversations between him and Candace.
Ferb is aware of Candace's crush on Jeremy, and once after a roller derby rematch, Candace was driven backwards into Jeremy's arms, Ferb complimented him on catching her. There was also another time when he dressed and posed like Jeremy to catch the attention of Romantic Candace so that she could come down and then they would merge two Candace back into one.
Ferb has almost come to an agreement with Phineas in terms of attitude towards Candace. Some of their big ideas are intended for Candace specially. He and Phineas will also offer Candace a prominent role in their plans (even though it's just Phineas who finally tells her the fact).
Despite their limited on-screen interaction, it is made clear that Ferb loves Candace dearly, which can tell from his behaviors. He has helped Candace countless times and helps Candace build plenty of things, together with Phineas. He, along with Phineas, is always willing to help Candace whenever she needs it. When they glided along the snow gorges, Candace was bounced on the cliff and fell onto Ferb's head precisely. Though seemingly uncomfortable, Ferb overcame the following obstacles carefully with Candace embracing his head, screaming. Besides, Ferb and Phineas respected her contributions to their mother's birthday celebration and recorded her as she rehearsed her song dedicated to Linda; Ferb even lifted his thumb finger to praise Candace for her beautiful voice.
Ferb will also appear properly and add pleasure to the atmosphere when Candace gets involved in their projects. One instance is the moment when Candace was crowned "Queen Wahini of the Beach", Ferb helped her celebrate by singing the song "Backyard Beach".
In addition, when Candace was trying to take the traffic surveillance video disk back from Norm, he and Phineas helped her by chasing Norm on their bikes. Eventually, they came to a drawbridge, and Norm was caught in the middle as the bridge rose up. Ferb jumped off and towards the robot, but was thrown off to the side. Fortunately, he appeared unhurt at last. Ferb also tried to help cheer up Candace by telling a little joke which drove all persons except Candace burst out laughing at the comedy show they had arranged for her specifically.
Ferb also saved Candace several times. Most time he accomplishes the task with help from Phineas. He has saved Candace alone twice as well. One instance included when he saved Candace from plummeting into a canyon on a kiddie ride whose ride time had expired with slingshot, sending a quarter into the kiddie ride to reactivate it. He has also helped her get out of a damaged rocket and space walk to safety.
Ferb might look up upon Candace somewhat, according to Phineas, Ferb was impressed with Candace's bravery when she jump-started their rocket that had ran out of fuel.
Ferb is very close to Perry and seems to care for him as much as Phineas does, though he is not as obvious about it. Perry switches beds in the middle of the night to make it fair between the two. Perry might be the reason that he gets along so well with Phineas, as shown in "Phineas and Ferb:Across the 2nd Dimension" when he goes to the pet store with his family.
Steve the Chameleon
"Steve" is a Chameleon Phineas and Ferb found in their backyard. Steve seems to prefer Ferb over Phineas, who fed him lots of mushrooms. Ferb even gave a long speech to inspire Phineas and Isabella to keep searching for him. At the end of the episode, it is taken that they keep Steve as their pet.
Lawrence is Ferb's biological dad. Ferb helps Lawrence out a lot and loves him very much. Ferb has been heard referring to Lawrence as "Father", not "Dad" or "Daddy".
Even though Linda is not Ferb's biological mother, he treats her with a lot of respect and loves her as if she is his real mother. Unlike with Lawrence, Ferb has called Linda "Mum" on one occasion.
Isabella is a good friend with Ferb, perhaps because he's the stepbrother of her crush, Phineas. Isabella doesn't seem to have a romantic interest in Ferb like she does with Phineas. However, when Candace implies to Isabella that she may instead end up marrying Ferb, Ferb responds by giving a winking look in her direction, implying that he may have some affection for her, but he may have just be teasing her. Or, perhaps he just considers himself a "ladies' man". Ferb is aware of Isabella's crush on Phineas. When they were stranded on the island, Ferb listened to Isabella as she lamented that Phineas never notices her and offered her a handkerchief as she cried. Several times when the three are together, Ferb simply steps away to leave the two alone.
When Ferb is picking up plans for his and Phineas's latest contraption, he first meets Vanessa Doofenshmirtz. He develops a romantic attraction to her, which leads to a mix-up between the plans he was picking up and those that Vanessa was picking up for her father. When later asked by Phineas how he got the plans confused, Ferb responds "I was weak."
Ferb encounters Vanessa again while test-driving Meap's spacecraft. When he sees that he passed by her, he backs up the spacecraft and looks at her as if he were trying to flirt with her. She responds by smiling at him kindly. During "Vanessassary Roughness", Ferb repeatedly offers his help to Vanessa, even though he is initially rebuffed. She learns his name, and calls it out when she is in trouble. She even kisses him on the cheek at the end of their adventure.
When Phineas and Ferb take off in their new ship to travel around the world, Vanessa lands on its cockpit. She sees Ferb, and joins the group of kids for a short period of time. During their time in Paris, Vanessa complains about her dad being evil, and Ferb says "Well, if you love somebody, you have to meet them halfway." He sees a rose stand, but by the time he returns to his post with a rose, Vanessa had left with her dad, Major Monogram, and Perry in a hovercar.
In the episode "Act Your Age", we learn Vanessa and Ferb do end up together and their age gap doesn't seem to matter. We also learn that they going out to eat Ukrainian food at the end of the same episode.
Jeremy participates in many of the activities that Phineas and Ferb plan. Ferb helped Jeremy dance on a national television show by doing the dance moves for him. At the end of the competition, Jeremy admitted the truth about his dancing and gave Ferb all the credit.
Ferb has helped Baljeet in some of Baljeet's projects. He has also been seen to give Baljeet some sympathy when playing a video game, in which he could've easily beaten Baljeet due to his internationally ranked video game skills. Ferb let a pecking bird control his game while he himself was reading a book, therefore letting Baljeet win to save his "fragile nerdy ego". In the end, though, he refused to admit it to Baljeet.
Buford and Ferb generally get along well, considering the fact that Buford is a self-proclaimed bully. Buford tends to go along with Phineas and Ferb's Big Ideas, but on occasion calls the two out for his own pleasure. Ferb once used a Vulcan-style pinch on Buford, after he accused Ferb of calling him a shark.
At some point, Buford nicknamed Ferb "Bean Pole McGee".
Not much is known about the relationship because they do not interact much, but Irving once got a lock of his hair.