While the details are unclear (for now) Tenzin has said that he and Lin had been growing apart for some time before Pema confessed her love for Tenzin. Tenzin took the opportunity and married Pema. It seems that Lin still harbours some resentment about the situation, and it is therefore likely Tenzin's feeling of 'growing apart' was not mutual.
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There is no clear answer, but it might be announced in a future episode (maybe in season two or three)
There's Korra (obviously). Tenzin, who is Aang's son, will be teaching Korra air bending. Apparently he's married with 3 children. There's also Mako, a fire bender who's friends with Korra. Bolin, an earth bender is Mako's brother. Tenzin's 3 children are called Jinora,Ikki and Meelo. All three of them are air-benders. Tenzin's wife is named Pema and she is pregnant with his fourth child. Amon is the leader of the equalists, who are against bending. Hiroshi Sato is a wealty buisness man, his daughter is Asami Sato. There is also Lin, she's Toph's daughter.
It hasn't came out yet but the creators said every one who is on Avatar the last Airbender will be dead in Avatar The legend of Korra but she'll probably be in some flash backs
Chief Lin Beifong received the scar when she attempted to arrest her sister using metal bending. Her sister, also a metal bender, cut the metal line that Lin was using and the tension in the line caused the metal to whip backwards, hitting Lin in the face and causing the scars.
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