Summary

The relationship system is a functionality introduced in Sword Art Online and continued in ALfheim Online that implements the in-game mechanics to manage and complement the social contracts between players with extra interactions becoming possible.

Types of Relationships

Strangers	The default state before players have added each other to their friend lists
Friends	Players who have each other in their respective friend lists
Guild Comrades	Players affiliated to the same guild
Married	Players who are married in-game

Strangers

In Sword Art Online and ALfheim Online, the players who have not become friends are unable to see each other's nicknames. In Sword Art Online, they cannot see each other's levels, either.

In Sword Art Online, the messaging system allows sending instant messages between the Strangers if they are on the same Floor and the sender knows the name of the recipient. These messages are very limited in length and do not have a feedback on whether or not the recipient has read their message yet.

Friends

Players can become Friends if a player launches the Friends function in the game menu and sends the request to the other player, who immediately receives a pop-up window allowing them to choose to accept or decline the request.

In Sword Arts Online, Friends are able to exchange short instant messages regardless of whether or not they are both on the same Floor. There also exists a quest reward item, Ring of Angel's Whisper, which allows its owner to send a voice message to their registered Friend once per month. They can also search each other's location on the map, and keep a separate shared inventory.

Guild Comrades

In Sword Art Online, belonging to the same guild offers to have the party's stats increased when the guild members are adventuring together, as well as the same benefits as registering everyone as Friends, including a separate guild-wide shared inventory. Additionally, the player gains the guild's insignia beside their name.

Marriage

In Sword Art Online, but not ALfheim Online, players can become Married in the same manner as becoming Friends, whereupon a player launches the Marriage function in the game menu and sends the request to the other player, who immediately receives a pop-up window allowing them to choose to accept or decline the request. It is not clear but is hinted at homosexual Marriage being allowed.

The Married couple can review each other's stat window whenever they like, and their personal inventories are merged into one that is shared between them and has the storage space amounting to the sum of their individual ones. The wedding ring then appears on the ring finger of each Married player which cannot be removed by any means apart from Divorce function.

While Marriage does not exist in ALfheim Online, the avatars who had been Married in Sword Art Online and thenceforward have been transferred to ALfheim Online do retain a hidden Married status.

Divorce

The functionality of Divorce is present in Sword Art Online as well. It has its quirks and consequences when invoked; if the players wish to Divorce after having become Married they must agree on how to split their shared inventory first, and the following options are offered to them: auto-allocation by value; auto-allocation by percentage; individual item selection. If they fail to agree on the split, the Divorce cancels.

An unconditional Divorce is possible if one of the players chooses 0% for themselves and 100% for the other, or if one of the players dies. In this case, the items that do not fit in the retaining player's inventory are dropped at their feet.

Trivia

• In Sword Art Online, few would become Married even while being a close couple for the reasons of possible scam, the disbalance between the male and female player numbers, as well as standing the risk of getting murdered for the extra items that drop by the widow or widower for grabs (as it does not seem that PK allows the murderer to loot the dead player).
• Main characters Kirito and Asuna are the most often seen Married couple in Sword Art Online.
• In SAO Progressive, having in-game sex emulation support is mentioned as a possibility, provided the engine settings are tweaked accordingly. It is unclear if the original Sword Art Online had the emulation working; the World Seed-based games do not. The function does not seem to be affected by Marriage.

References

Relationship System - Fan Wiki