Carl takes time off from his duties in the middle of a very important operation to send Rico a psychic message that helps him rescue Carmen despite only having seen them once since they left for the military and it wasn't under great circumstances.
The little speech Carl gives about how things are finally starting to look up in the war.
Carl: We've got one of their Brains now. Pretty soon we'll know how they think, and then we'll know how to beat them. One day it will all be over, and everyone will forget that this was the moment. This is when it turned. And it wasn't the mighty fleet, it wasn't a fancy new weapon. It was a drill instructor named Zim, who captured a Brain.
Zim asking Breckenridge if he is alright after breaking his arm in a sparring match. As well as later on demanding to the major that he goes to Big K with his men. Zim may be a Drill Sergeant Nasty, but deep down he cares about his soldiers. Zim ends up busting himself down to private and captures the brain bug.
Another prominent example would be when Rico gets Ten Lashes, for having Breckenridge killed by accident in a live fire exercise. Zim's the only one that talks him through it, even giving him a stick to bite down on in order to deal with the pain. It's implied that he went through the same punishment himself.
Zim: Bite down on this son. It helps, I know.
Rico drops out after a bad call on his part got another recruit killed, only to learn soon after that his entire family has been killed in a Bug attack on his hometown. When he goes back to change his mind, he's told he can't, because he already signed his discharge papers, and letting him go back on that wouldn't be legal. Sergeant Zim holds Rico's discharge papers in front of him and asks if that is his signature, and Rico, with no small hint of resignation, agrees that it is.
After eight chapters of not even allowing his wife to talk about him, much less write to him since his enlistment in the military, Johnny's father reunites with his son as a soldier himself in the Mobile Infantry in a big, tearful embrace of pride and love.
Just before Rico goes on his OCS practical assignment as a 3rd Lieutenant, the OCS Commandant informs him that Lt. Colonel DuBois, Rico's high school teacher, had written to him and asked that the pips he wore be given to Rico. Rico is touched, especially as he and DuBois had never gotten along in school... until the Commandant regretfully informs him that the pips are unavailable due to a real estate deal.
He's then offered a set of unlucky pips, where the last four of five cadets to wear those pips have flunked out. He swallows, accepts the offer to try and break the bad luck curse... and the Commandant thanks him, as he started those pips, the first to wear them as a 3rd Lieutenant.
Towards the end of the book, Rico is on his OCS practical assignment as a 3rd Lieutenant, leading an MI section into a Bug nest. His Platoon Sergeant is missing. Torn between leading his section to look for his man, and continuing on mission, Rico orders the Section Leader to continue the mission while he goes alone to make a one-man search. As he drops down into a hole and starts walking alone, he hears the Section Leader taking roll of the men on the radio. And then he hears the last order: "Section! By squads! Follow me!" And then the entire section jumps in after Rico.
Especially since Johnnie was a temporary 3rd lieutenant risking his chance at a permanent commission by breaking off to look for his sergeant, paralleling a story the OCS commandant told of another junior officer from the age of Wooden Ships and Iron Men who was cashiered for making an unauthorized pickup. He ordered his men to go on so that only he was risking his career but they followed him any way out of loyalty.
And this wasn't even Johnnie's usual unit. He was only in temporary command for his officer's test, the men had never dropped with him before and likely never would again. Johnnie won them over because of No Man Left Behind.