Krsna is not a different personality from Narayana. They are almost one and the same (both God) except Krsna has 64 qualities whereas Visnu/Narayana has only 60 of Krsna's 64 qualities (93.75%).

Krsna has 4 more qualities than the Visnu-tattva's have-

(1) Krsna is the performer of wonderful varieties of pastimes (especially His childhood pastimes.

(2) Krsna is surrounded by devotees endowed with wonderful love of Godhead.

(3) Krsna can attract all living entities all over the universes by playing on His flute.

(4) Krsna has a wonderful excellence of beauty which cannot be rivaled anywhere in the creation.

These four qualities above out of Krsna's 64 qualities is important to understand because as said, not even Visnu/Narayana forms of God, including Balarama, have them in full, although sometimes exhibited partially.

Krsna is the original Personality of Godhead, His first expansion is Balarama, who expands the original catur vyuha or quadruple expansions,

1 - Vasudeva 

2 - Sankarsana 

3 - Pradyumna 

4 - Aniruddha

These original catur vyuha expansions reside in Mathura and Dvaraka.

From them twenty four forms of Visnu expand. They are named differently according to the arrangement of the conch, disc, lotus and club in Their hands. 

All of these twenty four forms reside in each Vaikuntha planet with the predominating Deity of that planet. 

From the original catur vyuha the second catur vyuha expands. In this second catur vyuha, the form of Sankarsana is also called Maha Sankarsana. 

It is from Maha Sankarsana that Maha-Visnu becomes manifested who creates the material worlds (unlimited Brahmanda universes)

Lord Visnu/Narayana refers to a category of living entities known as Visnu-tattva, or expansions of God (Krsna).

There are billions of different names of Visnu, but the basic is “He who has entered everywhere”, as God’s omnipresence known Kshirodakashayi Visnu or paramatma.

Within the category of Visnu-tattva are included all of the unlimited incarnations and expansions of God that all have 93.75% of Lord Krsna's qualities.

There are some expansions that also bear the particular name “Visnu”, three of which are -

Karanadakashayi Visnu (Maha-Visnu).
Garbhodakashayi Visnu, (Hiraņyagarbha) 
Kshirodakashayi Visnu, (Paramatma or Supersoul)

The Karanodakashayi Visnu (Maha-Visnu) is the first Purusha avatar who is lying on the causal ocean (karana-udaka).

From him the innumerable universes (Brahmanda's) manifest as he exhales. And into Him the universes are destroyed as He inhales.

He expands deep inside of each Brahmanda universe as Garbhodakashayi Visnu. 

This Visnu is lying on the universal waters that originate from His perspiration, and which fill the universe halfway.

In his navel is a lake, and from that lake grows a lotus stem. Lord Brahma is born within that lotus. Inside the stem of the lotus are the fourteen planetary systems.

Garbhodakashayi Visnu (Hiraņyagarbha) expands as Kshirodakashayi Visnu (Paramatma) who resides in Sveta Dvipa on the milk ocean and in the hearts of all living entities in all material universe.

Sveta Dvipa is a Vaikuntha planet that the Lord manifests within the material universe.

In reality it is more like a window through the material covering, which allows one to see a portion of the spiritual realm. 

It is an eternal abode, and it continues to exist even after the destruction of the material universe.

The system of the Lord’s incarnations is that all incarnations come through the form of Kshirodakashayi Visnu in Sveta Dvipa.

Lord Brahma along with the material administrators or demigods stand on the shore of the milk ocean and pray to the Lord to incarnate. They are not able to see the Lord, but Lord Brahma receives confirmation from within his heart.

Factually there is no difference between one Visnu form and another, as they all belong to the same category of Vishnu-tattva (93.75% of Krsna's 64 qualities) but still known as the Supreme Person.

Krsna is not a different personality from Rama, and Rama is not a different personality from Visnu/Narayana. 

They are almost one and the same except Krsna has 64 qualities, whereas Narayana/Visnu has 60 qualities.

Krsna therefore has 4 extra qualities than Visnu/Narayana form

The category of Visnu-tattva takes millions of different Forms according to the mood of the devotees approaching the Lord.

There is no higher and lower capacity among the Visnu-tattva expansions and forms of Lord Krsna.

Although Lord Siva is in a category of His own and is neither jiva-tattva (jiva-soul) [78.125%] or Visnu-tattva [93.75%]

Siva's qualities are 85.938% of Lord Krsna's 64 qualities whic means he has 55 of a

Some Visnu-tattva's (God) choose to withhold certain aspects of their opulences to play a certain part within the material universe, for example, if a suitable jiva-tattva (jiva-soul) is not found in the material creation to become the next Lord Brahma, then Lord Visnu/Narayana descends and takes on that role of Lord Brahma.

Many gopis (Vrindavana village girls and women) and gopas (Cow herd boys) are both jiva-tattva (78.125% having 50 of Krsna's 64 qualities) and Visnu-tattva (93.75% having 60 of Krsna's 64 qualities) who play a simple role in Krsna lila.*^