The answer of the eleventh riddle - Profound Representative
The first line Cards of destruction
» It's the 'cards'. Plural...
The second line The king was thirteen
» 'The king' represented 'thirteen'.
The third line The joker came with seven
» And 'The joker' came later, along with 'seven'.
The fourth line And I was the highest of all
» The highest of all cards. Could mean the deck of cards...the playing cards to be exact.
- The fifth line The one which was spared
» The said card was 'spared', meant the others were not...
The sixth line The symbol of death and mysteries
» The card represented the death? and the mysteries?
The seventh line "I've been holding it in my hands...
» Someone was holding well...the card.
The eighth line ...Believed that he will be by my side and protect me..."
» Indicated that this card also represented someone, and it's a "HE".
The ninth line What was I?
» Well...the question.
When you think about it...this one was very easy...
First the cards referred to 'the playing card'. And if you thinking about it in DC, you would instantly recalled the happenings in the second movie: The Fourteenth Target.
'The king' of spades was referring to our chubby inspector, Megure Juzo, which 'Juzo' has the kanji of 'thirteen'.
'The joker' and the 'seven' of spades was found along with the stabbed body of Osanai Nana. 'Nana' is the 'seven'.
And 'The ace of spades is traditional seen as the highest card of the deck of playing card, which referred to Shinichi, as 'Ichi' is 'one'. This card is also known as the 'death card', which funnily, suited him best XD
(And as in RainbowKittyRune's review, the death could meant those people who were killed and the mystery was the identity of the culprit...too.)
Let's move to the next riddle, shall we?