All credits to koalasan_z 
Some of you may know this already but it might be a help to some.
I didn't figure this out myself until I watch this YouTube video where it's explained (It's in Japanese).
Why is this important?
>> A big part of Dragon Ball Z Dokkan Battle are the Link Skills. Having characters together that activate these link skills is very important to win battles (this is probably the most important, ex: Events Dokkan.)
>> Another reason is to have a character that can defend on the turn you want. If you have 2 or more characters that can defend, you can guarantee a defending character on each turn (there are situations where it won't work, will explain later).
How it works?
>> Ki Spheres when entering battle = Random.
>> Allied characters order when entering stage = Random.
>> Rotation of allied characters order after entering stage = NOT RANDOM!!
As you can see, we can't manipulate the order of allied characters when we enter a stage but after that, there is NO randomness involved in the order that the allied characters appear in battle.
>> Notice the character dial at the bottom right of the battle screen. Every time you enter a stage, this dial is randomly generated.
>> When entering a battle, the first 3 characters starting from the 1 o'clock position of that dial will start the battle. If you DO NOT reposition characters during the fight, they will appear in a sequence, as followed:
|Char. Position / Status||Turn 1||Turn 2||Turn 3||Turn 4||Turn 5||Turn 6|
|Pos. 1 / In battle||Char. 1||Char. 4||Char. 7||Char. 3||Char. 6||Char. 2|
|Pos. 2 / In battle||Char. 2||Char. 5||Char. 1||Char. 4||Char. 7||Char. 3|
|Pos. 3 / In battle||Char. 3||Char. 6||Char. 2||Char. 5||Char. 1||Char. 4|
|Pos. 4 / Next in battle||Char. 4||Char. 7||Char. 3||Char. 6||Char. 2||Char. 5|
|Pos. 5 / Next in battle||Char. 5||Char. 1||Char. 4||Char. 7||Char. 3||Char. 6|
|Pos. 6 / Next in battle||Char. 6||Char. 2||Char. 5||Char. 1||Char. 4||Char. 7|
|Pos. 7 / Not in battle||Char. 7||Char. 3||Char. 6||Char. 2||Char. 5||Char. 1|
Notice that characters in 'Pos. 1' and 'Pos. 2' will return in the battle in 2 turns but the character in 'Pos. 3' will return in 3 turns.
That means that every character that you leave on the 3rd position will take 1 extra turn to return in the battle.
Note: During the stage, if a battle ends on Turn 5, this sequence of characters does not change (considering Table 1); the next battle will start with 'Char. 2', 'Char. 3' and 'Char. 4'.
Table 1 in-game illustration
Turn 1 Turn 2 Turn 3
Turn 1 Turn 2 Turn 3
Let's say 'Char. 1' and 'Char. 2' have a really powerful link skill and I always want them together on the field.
When looking at Table 1: on Turn 1, 'Char. 1' and 'Char. 2' are together, on Turn 3 they are still together but on Turn 5, only 'Char. 1' is present.
How to fix this? Well, as explained before, characters in 'Pos. 1' and 'Pos. 2' will always appear together in 2 turns. That means that, on Turn 3 you have to reposition your 'Char. 1' and 'Char. 2' to 'Pos. 1' and 'Pos. 2', for them to stay together on Turn 5.
>>> REPOSITION TURN 3 <<<
As you can see, 'Char. 1' and 'Char. 2' will appear together on Turn 5 now.
So, in this case, pairing 'Char. 1' and 'Char. 2' in 'Pos. 1' and 'Pos. 2' will always keep them together.
Of course, there are cases where you will consider breaking this link if, let's say, 'Char. 2' can defend against the enemy that will attack on the character in 'Pos. 3'.
Now, let's say 'Char. 1' and 'Char. 4' have a really powerful link skill but 'Char. 1' appears on Turn 1 and 'Char. 4' appears on Turn 2. Without re-locating them, they will never appear together on the battle field.
How to get them on the same turn? As mentioned before, characters in 'Pos. 3' will come back in 3 turns opposed to 2 turns for characters in 'Pos. 1' and 'Pos. 2'. That means that we can shift the character in Pos.3 from appearing on an even turn to an odd turn and vice versa.
In this case, on Turn 1, we want to move 'Char. 1' to 'Pos. 3' because we want to move him to the even Turns where 'Char. 4' appears.
On Turn 2, we want to keep 'Char. 4' on either 'Pos. 1' or 'Pos. 2' because we already shifted 'Char. 1' and shifting 'Char. 4' would mean that they won't meet each other on the battlefield again.
>>> REPOSITION TURN 1 <<<
As you can see, 'Char. 1' and 'Char. 4' will appear together on the battlefield on Turn 4.
Now, all you need to do is to keep 'Char. 1' and 'Char. 4' in 'Pos. 1' and 'Pos. 2', as explained in Case 1, and they will always appear together on the battlefield.
The character dial on the bottom right does not re-order itself to the new character sequence during battle. It will only highlight the characters that are on the battlefield.
Once the battle is over, the dial will re-order itself to the current sequence.
Things to remember!!
>>> Characters in 'Pos. 1' and 'Pos. 2' will always re-appear in battle in 2 turns.
>>> The character in 'Pos. 3' will re-appear in 3 turns.
>>> Characters in 'Pos. 3' will shift from an even turn to an odd turn and vice versa.
What you normally want to do is to try and rearrange your characters just before a boss fight, during the pre-fights if possible. You might not have the luxury to rearrange your characters during a boss fight as some of them hit really hard from Turn 1.
If you're really confused because of my awful writeup (edit : awful standardization), the best thing to do is to open a spread sheet and just make your own table and experiment.
I hope this helps some of you, in understanding how the character sequences in battle work.