ok i've battled a lot of DKs, and good ones too and i'll say this.
DK's HATE Ike. Ike's big disjointed hit box really gives Ike and advantage here. and the fact that DK's recovery is one sided like Ike's means they can gimp each other. Jab is what DK hates about you the most. As it is so fast and they can't do anything about it. Alot of DK's try to shield grab your fair so Make sure you maximize your spacing because DK has a good grab range. And getting grabbed = death for you once you at higher %'s. He can stage spike or just cargo down throw you off the stage.
BUT, If you can land the aether spike on DK, he is NOT coming back at all, he has NO vertical recovery. I've killed many DK's this way. DK will also ledge stall with his Up B till you get too close then he'll ledge attack you, if 100% that is. Also, if he is steaming (fully charged DK punch) then he'll attack you with that off the ledge sometimes. Counter works wonders on that. Forward tilt is great in this match in my opinion. its good for finishing off DK since DK is pretty fast. But when it comes to speed, Ike's jabs are the fastest in this match up.
DK also likes to approach with his Bair which can auto cancel and be done twice in a short hop, yep its that fast. He has the fastest aerial of you two. But, fair has more range so if you exploit that you can knock him out of it. Best to shield than jab when on the ground, but watch out DK can WOP you with it and gimp you with it. You might want to peak aether (Aether when the peak is at the ledge) To not get gimped.
DK also tries to approach with Up B super armor frames. Be careful but jab cancels it out once the SA frames are over, (which when he starts spinning). I normally approach by air though so i end up jumping over it and just fair then out of it. If a DK recovers too high with his Up B, you get a free forward tilt in. yea, Dk has bad post lag when landing.
One 2nd last note, the DK punch is much more stronger than fully charged when its at 7-9 wind ups. (the number of times DK spins his arms around). It does more knock back and kills you by 50%. So be on your guard, but the flaw in this is the the punch does NOT have super armor frames. DK's who don't use SAF though will love this technique and will abuse it. But note it takes longer for them to use it making it very counterable once you get the hang of it.
My final note is that DK and Ike kill each other at the same speed generally. His forward smash kills you by 100% sweetspoted (his hands)... (ok...stage dependable) Your kill move in this match is forward tilt. That kills by 100-110%. (stage dependable) If you land forward smash, (which can hit DK from behind cause of his size, so a roll happy DK is prefect prey for this) it kills DK at 80%. Use fairs and nairs, but i think drop the nairs when DK is near the edge and your at a %'s. You don't want to get shield grabbed and stage spiked, kills you at 45%. jab is DK's worst enemy. Bair is a good finisher and follow up to some moves.
55-60% cause its all on who can kill. But Ike's disjointed hitbox and jab is an edge in getting the damage first.