On the DB page it said-
Characteristics are another in-game barometer of key strengths. In your Pokémon’s summary it will say one of the following messages, which tells you your Pokémon’s best IV stat. For example, if it says ‘Proud of its power’ then its Attack IV is higher than the other IVs.
But if a Pokemon has all perfect IVs, how will the Characteristic be determined.
Pleaselog inorregisterto add a comment.
It wouldn’t be a random characteristic but one from this list:
Often dozes off (HP)Likes to thrash about (Att)Capable of taking hits (Def)Mischievous (Sp. Att)Somewhat vain (Sp. Def)Alert to sounds (Speed)
This is because these 6 characteristics all have 31 as one of their possible iv values. Therefore, because all stat ivs are 31, the game would choose one lf these characteristics at random.
The characteristic will be a random one.
Source: I have 2 6 IV Chimchars and a 6 IV Skarmory as well as a 6 IV Ditto in my PC right now.