Tags describing how characters are related to each other. This can be genetic (family) or non-genetic (non-family).
Here is the non-exhaustive list of the various relationship tags on e621, or add a new tag in one of the categories below: Non-Genetic
Characters that are not related by blood/family. Such as neighbors, co-workers, etc.
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Profession
Housing
Ecological Relationships
Power Exchange
Non-Gentic: Other
Family
Characters related by family.
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First-degree
Child - currently used for all young characters, not just depicted children
Second-degree
Other
Paired relations
Not necessarily explicit.
Explicit
Family: Other
Meta
Relation between characters based on outside information. Such as crossover, body swaps, etc.
These relations typically wouldn't exist in reality.
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Related Tag groups