Increasing Yelan's Friendship Level in Genshin Impact does not increase any of her combat skills in the game. However, they do unlock her stories, voiceovers and character name cards for those interested in lore. All these can be achieved by farming Companionship EXP in the game daily.
Although increasing the friendship level is not the game's main objective, veteran players always farm it as they have more free time after finishing their Resin.
Here are some methods through which players can farm for Yelan's Companionship EXP in Genshin Impact.
How to reach Yelan's Friendship Level 10 quickly in Genshin Impact
1) Complete Daily Commissions
The most obvious method to increase Yelan's Friendship level is by completing Daily Commissions. Players can earn up to 60 EXP per commission and 100 EXP every day by talking to Katheryne, depending on their rank.
While fulfilling the commissions, remember to add Yelan to the team or she will not receive any Companionship EXP.
2) Use Resin for Domain and Leyline
Genshin Impact players can also farm Yelan's Companionship EXP by spending Resin on any available Domain or Leyline. However, the amount of EXP given depends on their Adventure Rank.
Ley Line Outcrop (Blossoms of Wealth and Blossoms of Revelation) can only give out a maximum of 20 EXP. Meanwhile, normal and weekly bosses grant up to 45 and 70 EXP, respectively.
3) Join Co-Op for double EXP
In the co-op mode of two players, the number of Companionship EXP earned by characters doubled. This is due to the fact that players can only have two characters in their party, and the reward is split between the two instead of the four.
However, players must note that a co-op of more than three people will not double the EXP.
4) Serenitea Pot
When players keep Yelan in their Serenitea Pot as companions, they gain EXP based on their Adeptal Energy and Trust Rank. Every hour, a total of five Companionship EXP is distributed at 12,000 Adeptal Energy.
Thankfully, the maintenance for the Serenitea Pot ended on May 31, allowing players to add characters to their Serenitea Pot as usual.
5) Trigger Random Events
Random Events in Genshin Impact can assist players in gaining Companionship EXP.
The 'Friend to Animals' Random Event is one of the best events players can find in the current version. It can be completed 10 times each day, and players can earn up to 150 EXP per day.
It is even better if Travelers can do it in co-op mode as they will receive 300 EXP per day instead.
All the methods above can be used to quickly increase Yelan's Friendship Level (or that of any other character in Genshin Impact).
Upon reaching Friendship Level 4, Yelan's story in her character interface mentions the newest Fatui Harbinger, the Regrator. More information regarding this figure is available in this post.