Essential is a synonym of Critical. Critical is related to Crucial. Crucial is the opposite of Light. Light is a different concept from Shadow. 
Reliability is a type of Virtue. Virtue is related to Merit. 
Essential <--Synonym--> Critical <--RelatedTo--> Crucial <--Antonym--> Light <--DistinctFrom--> Shadow 
Reliability --IsA--> Virtue <--RelatedTo--> Merit 