Baatar was hired by Suyin to be his architect at one point. Together, they built the city of Zaofu in the Earth Kingdom where the Metal Clan would live. However, Su still felt unsatisfied like she was missing something. Su later realized that was she really wanted was a family. At some point, she and Baatar fell in love and got married. They ended up having five children together. Baatar would often skip meals to work in his office and Suyin was supportive of this to which Baatar was thankful. The two worked well when making decisions with their children and both agreed to allow Opal to join the new Air Nation.
Baayin is a popular ship due to the fact that it is canon. By the time that the series started, they were already married with five children. Before Suyin was revealed as a character, many thought that Lin was the only child of Toph. Since Lin was not married and did not have any children, many thought that the Beifong family would die off after Lin. This is why many fans were excited when Su was introduced with her husband and family.
Baatar Jr. is the eldest son of Baatar and Suyin and is (presumably) a non-bender. He is named after his father and looks a lot like him. Baatar Jr. is also a genius inventor and worked along with his father before he left Zaofu. After the Earth Kingdom fell into anarchy and Su refused to get involved, Baatar Jr. decided to leave with Kuvira to help her form the Earth Empire and the two got engaged at some point. He even willing threw his father and brother Huan into prison when they refused to bow to Kuvira. However, she later fired a weapon at him in order to kill her enemies so Baatar Jr. eventually turned on her.
Huan is the second son of Baatar and Suyin. He is an earthbender and a metalbender. However, he does not really use his bending to fight. Instead, he is an artist and often uses his metalbending to create sculptures. Huan also seems to enjoy painting and working with Ikki and Meelo on paintings. At some point, Zaofu was captured and Huan was imprisoned after refusing to bow to Kuvira. He was eventaully freed and fled to republic city.
Opal is the third child and only daughter of Baatar and Suyin. She was born as a non-bender but became an airbender after Harmonic Convergence. Opal was rather shy but was excited to meet her aunt Lin and wanted to get to know her. Opal also fell in love with Bolin and the two eventually started to date. Lin eventually convinced Opal to join the new Air Nation which she greatly enjoyed.
Wei and Wing
Wei and Wing are the youngest twin sons of Baatar and Suyin. They are both earthbenders and metalbenders. Wei and Wing are very playful and even invented a metalbending game called Power Disc. They are also very loyal to their mother and often work with her when needed. They helped her and her sister Lin sneak down through the dome when the Red Lotus captured Korra. They also helped her in attempt to assassinate Kuvira but were captured along with her.