These are all based on their in voice lines which can be found in the game or here!:

• Barbara and Venti are friends! despite what happened to the holy lyre. Barbara comments that Venti has a really amazing voice while Venti says her voice is really sweet. Implying they either sang together or watched each other perform!
• Xiangling used to be part of Beidou’s crew! Beidou and her crew often misses Xiangling’s cooking while Xiangling has nothing but admiration for Beidou. Beidou often gets her fresh seafood ingredients which she experiments on
• Razor is a student of Lisa! Lisa thought him how to control his vision and seem to be quite fond of him. They often do combat practice together even though Lisa doesn’t like doing work she puts effort on teaching razor 
• Chungyun doesn’t like Xiao that much even though he respects him a lot for what he has done for Liyue. Xiao’s attitude towards mortals puts him off. This also implies that chungyun has at least have either met him or knew him through someone else that had direct contact 
• Apparently, a lot of characters have a bad impression of Ningguang, most notably xiangling, xingqui, and keqing. Beidou seems to have a negative impression of her as well but doesn’t really care. This makes her the most disliked playable character in game so far 
•  A lot of Knights of Favonius members seems to either be smitten or respect (or both) a future playable character named Albedo (the head alchemist of the knights.) Kaeya and Lisa remarks that he is quite good looking while the rest respects him a lot (Klee remarks that he is her friend). Venti is wary of him though
• The character with the most voiced lines about them is Jean! Jean is the most respected character in Mondstadt while Zhongli in Liyue. Funnily enough, characters who have lines about Jean have met her while characters who have lines about Zhongli haven’t or me him at least once 
• Xiao is the only character who is mentioned by Qiqi in memory format. Since Qiqi has no memories of her previous life and have trouble remembering things for a long time (even the face of her caretake), this may imply that Xiao might have left a strong impression on Qiqi before she died. 
• Venti and Mona know each other! Venti remarks that both have similarities of having jobs that would make a person poor (bard & Fortunetelling). Mona doesn’t dare to read fortune abouts Venti due to a previous incident 
•  Additional:  Mona asks the traveler where Paimon comes from and says: I cannot seem to discern a single shred of information regarding this floating fairy’s fate meaning that Paimon most likely isnt from Teyvat 
• Noelle loves spending time with Barbara and vice versa! They have tea breaks together sometimes, Barbara usually hums/writes songs while Noelle makes snacks for them. Barbara comments that Noelle is just the cutest! 
• Diluc has the most “bad impressions” in mondstadt due to his status of being an ex knight which put a lot of strain on his relationship with most of the characters. Though Barbara thought Diluc might have disliked her performance due to him frowning at her one time 
• Kaeya is probably the character with the most suspicion directed at. While he is praised and respected for the skill as a knight, characters like Diluc (obv), Amber and Sucrose are wary of him because of how he acts 
• Varka, The grand master of mondstadt seems to be respected a lot by the knights, most notably Jean and Kaeya. Diluc on the other hand seems to dislike him a lot and hinted to be one of the reasons why he left. He was also the one who gave Razor his name 
• Xiangling, Chongyun and Xinqui are the only 3 characters that all have each other in their own voice lines with a positive reaction. Xinqui drags Chongyun to eat with him at Xiangling’s resto very often, most likely to poke fun at him for not handling spicy food well.
  
• Barbara is really good friends with Lisa! and says that if Lisa ever becomes a star she’ll support her until her voice turn hoarse.
  
• While Ningguang is currently the most disliked PLAYABLE character the most hated character is actually someone called Hu Tao! Xiangling, Xinqui, Chongyun and Qiqi (especially) despises them for their overall attitude, pranks & jokes. Howeve Xiao seems to think that she is funny.
  
• Ganyu, the secretary of Ningguang has a small assistant named Yaoyao who is a dendro user! Ganyu, Ningguang and Beidou dotes on her a lot (though Ningguang doesnt like to show it). Yaoyao is also a fellow chef and a junior of Xiangling!
  
• Chongyun & Xinqui seem to have a mutual distaste for a character named Xinyan! Nothing on a personal level, it’s just because she plays rock music (which disturbs Xinqui’s reading) and her concerts are filled with flames (which bothers Chongyun). Xiangling likes her though!
  
• Xinqui and Xiangling are the only characters from Liyue to have voice line for a character in Mondstadt! (For now) Though both characters are yet to be released (Albedo and Dionna). On the other side, Venti is the only one who has a voiceline for a character in Liyue (Morax). If all of Xiao’s line from the closed beta are added in canon then he would be included for his line on Venti!