623C beats out ID, so using it on his wake up forces them to block and respect you. It's also nice to use this when baiting out his ID's and make him scared to use it as often (Although blocking works just as well in this case). Remember that Jin has a good DP as well in this matchup, although Ragna does have the tools to space and bait it as well.
In terms of lose combat, Ragnas will sometimes whiff 2B or 2C after 5B or 5C to gain momentum, so look out for this and punish him with a 5B or 5C after it too. It's one of his best ways of gaining momentum in a close quarters situation to do heavy damage. Your jab is also better than his, so try and punish him to a combo when he starts using frame trap and stalled 2As.