Pippin is a massive (literally) brother figure towards Rickert, the youngest member of the Band of the Hawks, and will protect him with brutal ease as one unfortunate enemy solider's skull found out when he attacked Rickert. Pippin is also protective over Casca, dragging her away from Guts and Griffith's duel so she wouldn't get hurt or interfere. This ultimately culminates in the Eclipse, where he holds off a veritable army of demonic horrors to buy Casca and Judeau time to escape, at the cost of his own life.
Judeau seems like this toward Casca as well when a few Chuder Soldiers tried to rape Casca, Judeau has some choice words "She's our only flower you'll pay a high prince for what you've done". Judeau also gives her advice when it comes to mercenary's life and sacrifices himself to save during the Eclipse but then it's also revealed he had some non-brotherly feelings for her before he dies.
Sister version with Casca and Corkus; despite him being older than her, Corkus treats Casca like this and expressed it when she saved him from Guts early in the story though she said it was his fault for attacking Guts. Corkus even outright calls her "big sis Casca".
Guts has this with every younger male friend, Ricket frequently looks up to Guts due to the latter protecting him in battle. The only thing that stopped Guts slicing Griffith upon his rebirth was the fact Ricket got in his way though Guts compromised by shoving Ricket safely out of the way and continuing his revenge. Isidro gets both Big Brother Mentor and Papa Wolf from Guts, and though neither would admit how much they care, but it’s shown when Isidro runs away from the group when upset and Guts follows him secretly to keep eye on him.
Guts also plays big brother with every human female with the sole exception of Casca. Princess Charlotte, Jill, Schierke and Farnese on the other hand get the big brother treatment from him, in Charlotte and Schierke in particular even getting piggy back rides from him.
Though he's only a year older than her, Serpico is this over his half-sister Farnese and will not accept her being in any danger, fighting demons and even Guts to protect her. Serpico goes beyond simple brotherly care as he believes he must make Farnese happy as well and when Guts's arrival changed Farnese's life Serpico even contemplates killing Guts out of jealously before accepting it. There's also other feelings between Serpico and Farnese that go unsaid.
